SteeMan

Agree! I am sick and tired reading this every fu***** day: "we want Armbian on XXX Please help", "i need instructions how to ..." "do this" There is a forum https://forum.armbian.com/forum/22-board-bring-up/ full of insane demands like "make Armbian on this and that board", but nobody asks themselves how much does this costs and who (will) pay(s) this shit ... !?
 
The other day one come to IRC starting to ask me if I can explain him how TV boxes boots. I said, we don't support that, but he didn't take this as an legit answer. And the more I explained that I don't know and even I knew I would not write manuals because he asked, the more aggressive he got ... he - while abusing my time - accused me (!) for hiding the information about boot processes ... amazing.
 

I estimate core crew makes 30-50h every day to keep this project running which means even with 10.000 USD per device we are not covered, but we can at least silence insane wishes and demands.
 
We have to work on this problem properly.

Everyone likes to say one word "give" and always for free (all free). Give a new core, and all the functions will work in it. Give support to all the existing equipment in the world and make sure that it turns on automatically, without any settings. Give me a new loader. Give the installation system to eMMC and so that you need to press one button, and better from a mental order. Give full-screen 4K video playback on any shit. Give the perfect job to all the shit that is produced and sold for $ 10-20. Give the source code, and that they would work immediately on the same command and without errors. Give answers to all your questions and be sure to answer everyone personally and in detail. And most importantly, everyone wants it for free and immediately. For everyone who wants to get it-there is a tariff, pay $ 10,000 and you will get full support for your TV box model for 1 year. 

The build you have installed is built from balbes150's fork.  You don't mention which git tree you are working with, but you should be working with https://github.com/150balbes/Build-Armbian
I believe balbes150 just removed the s905xxx targets and consolidated into the generic arm-64 target, so if you use the current head of the git master branch you will be working with newer kernels.  I pulled this code yesterday and built Armbian_20.05.1_Arm-64_bionic_current_5.6.2_desktop.img and in output/debs is the headers package linux-headers-current-arm-64_20.05.1_arm64.deb
It is unlikely that you will be able to rebuild the header package that corresponds to your currently installed build, but you can certainly build a current version with headers for you to work with.

Is your system clock correct? 

You do have access to the source for these builds:  https://github.com/150balbes/Build-Armbian and https://github.com/150balbes/Amlogic_s905-kernel (actually these git trees are usually a few days behind the most recent balbes150's builds as he waits for code changes to stabilize before pushing them back to these github trees)
I would recommend you start by pulling the Build-Armbian environment, and building the equivalent of what you currently have installed.  You will see that the build system creates the kernel header package as part of the build process.  Then you can experiment with improvements and contribute back to the overall community.

The standard armbian documentation only applies to the standard armbian builds.  The builds by @balbes150 are a fork of the standard armbian code for Android TV boxes (instead of standard SBC boards) and therefore you can't use the standard documentation as there are a number of differences.  To install into emmc look in your /root directory and you will see a set of install scripts.  Choose the one appropriate for your CPU and run it for installation into emmc.  The current active thread of discussion for the @balbes150 builds is: 
Please carefully read the first posting in this thread as it goes over many of the specifics that are unique or different for these android tv box builds.

You would repeat the process you just completed.  Create a new sdcard with the desired distribution, then copy that to internal emmc with the provided scripts in the /root directory.  If you boot the box with the sd card inserted, the box will boot from the sd card, thus allowing you to overwrite the contents of the internal emmc.

As I have been watching the development of Armbian for TV boxes, there is one area that my needs haven't been addressed yet.  And since this is open source that means I needed to read a lot of posts and get into the code to understand how things are working.  I have been doing that over the last six months or so and wanted to report back on what I am now doing to solve my issue as it may help others.
 
I have a number of TV boxes running as headless servers for a variety of projects.  Once I install Armbian on them I then get OS updates through ubuntu (I am using Ubuntu Bionic on my boxes).  That gives me bug fixes and patches for the software running on these boxes via apt update/upgrade.  However the linux kernel generally stays unpatched at the point in time that I installed one of balbes150's TV box builds.  What I wanted was to be able to build and update kernels so that I can apply new kernel versions to my servers.
 
I am now able to do that successfully across a number of TV boxes ( TX3mini, TX3 X3, H96 Max X2) (Note I only have amlogic based boxes that I test with).  I am building kernels from kernel.org sources, building .deb files and then installing those files across my servers as new kernel versions are released.  My goal was to simply stick with the 5.4 kernel as it is a long term support kernel, but I find that I have moved to 5.6 as I wanted native wireguard support.  Another goal I have is to do all this with as few patches as possible (i.e. running stock base unmodified linux).  And a final goal was to build natively on my arm based boxes.
 
Below are my notes that explain what I have done and hopefully provides others with some of the knowledge I have built up over time to be able to get what I needed working.
 
I hope this is helpful to others who want to get their hands into armbian a little deeper than just installing and running it.
 
 
build-debs.txt

H96 Max X2 (with S905X2 cpu) 4GB Ram ad 32GB Emmc
 
I purchased this box thinking I was getting an X96 Max (the naming of all these different boxes is very confusing).  But it turns out that it works well with current builds of balbes150's Armbian.  It works with the meson-g12a-x96-max-rmii.dtb.  It works well installed to eMMC.  The wifi does not work, and I haven't tried bluetooth or hdmi audio.

You will need to build the .deb files from source in order to upgrade.  I have been doing this for the last couple of months and things seem stable enough for this work fairly smoothly.  You will need to use the build environment fork from balbes150's git repository (https://github.com/150balbes/Build-Armbian).  It is quite simple to do a build from source if you have an intel based machine running Ubuntu18.04.  (Note there is one mistake in the instructions, when is says to "cd build" after "git clone https://github.com/150balbes/Build-Armbian", it should say "cd Build-Armbian")
Once you have built the .debs you can use apt to upgrade them in your installation.  Look at what packages you have installed and upgrade to the newer versions you have built.
I would recommend doing a image backup before you attempt the upgrade until you are comfortable with the process.  To make a backup, burn an Armbian image to SD or USB, boot from it and run the 'ddbr' command to backup the eMMC. Then you will have a backup image to restore from if something goes wrong during an upgrade.
 

I think you have run into the same issue I recently did.  I have purchased four TX3 mini boxes (2G/16G) over time from different vendors.  Two of the boxes work perfectly (i.e. I can install to the built in eMMC.  However on the other two boxes, I can not.  The two boxes that can't install to eMMC, do work in all other ways.  So I can run armbian from the SD card if I want.
 
In attempting to diagnose the problem, I noticed that the kernel wasn't detecting the internal storage at boot time.  So I opened up the cases of a box that worked and a box that didn't and discovered a difference.  The two boxes have different storage chips (the chips are actually located in different locations on the board as well).  In googling the part markings on the chips, it appears that the box that works correctly has an eMMC memory chip installed, and the box that doesn't has a NAND chip installed.  I recall reading somewhere that there isn't currently support in these kernels for nand storage.  So it would seem that some manufacturers in order to cut costs are installing nand storage on some of the TX3 mini's.  I don't know how to tell at ordering time, what type of storage might be used and therefore I no longer am purchasing the TX3 mini's for my purposes.  I really like the boxes and if I could be assured of getting boxes with eMMC I would use them for a number of projects.