Ian Coelho

So yes the emmc isn't detected (there is no mmcblk1).  Are you sure you have emmc on the box?  (I have for example some TX3 boxes that have old style nand instead of emmc and mainline linux doesn't support nand.  TV Box manufacturers will cut costs in anyway possible and nand is cheaper than emmc)  You would need open the box and inspect what physical chips are installed.
 
Since I don't have your box, there is little I can do to debug this.  Have you looked at the full log of the boot process to look for any indication of issues reported during the boot?  (You would need to find the debug serial console pins on the mother board and hook up a usb serial reader to capture the boot messages)  (some of this information goes to HDMI, but some gets output before HDMI is initialized).
 

I had this problem too, thank the user B1oHazard I solved it. The problem is the NAND frequency in the .dtb... some x96 mini have a different nand. To fix it, you need to change the .dtb file:
 
You need to unpack your dtb file into dts via device-tree-compile tool. Install it in linux ubuntu using 
sudo apt-get install device-tree-compiler decompile the dtb file:
dtc -I dtb -O dts -o meson-gxl-s905w-p281.dts meson-gxl-s905w-p281.dtb edit this file searching for "mmc@74000" block
max-frequency = <0x5f5e100>;
5f5e100 in hex = 100000000 in dec
edit it to 0x2faf080, 50000000 in dec
compile with
dtc -I dts -O dtb -o meson-gxl-s905w-p281.dtb meson-gxl-s905w-p281.dts  
for  Armbian_20.10_Arm-64_focal_current_5.9.0.img I prepared the file for you (see attachment)
meson-gxl-s905w-p281.dtb

@Ian Coelho How about some basic information first.  What build are you running?  What dtb are you using?  What dtbs have you tried?  How are you attempting to install to emmc?