Use Armbian Imager to create SDcard with curent Release. Connect HDMI Monitor and USB Keyboard. Put Sdcard into sdcard Slot and Power the Box. Follow instruction on Monitor

I ran memtester and found memorie errors, which is probably the cause of the problem I report med earlies. Hopefully this can be solved by changing voltage er timing settings.

A question from a beginner: how do I install it to use it for the first time? Should I use a flash drive and Rufus, or is there another way? I didn't quite understand, and I couldn't find any clear information.

Let me know. I had hopes the commiter would provide upstreaming effort proof but nothing there yet.

For a start it looks like you are wiring to spi0 and so fragment0 does not make sense. It also might be causing the pa14 pin problem as the op1 does not have an spi1. Pin pa14 is used on a uart output. I suggest you get rid of fragment0 and just use armbian-config to enable spi0.