I found a way to boot after having deleted the U-boot partition, which prevents any useful boot to allow repair. I could then complete the armbian install. This applies to my Pinebook A64, 14" (not the pro pinebook - i cant test that)
After removing the U-boot partition, it would only boot to a "busybox" with limited features, not enough to do a repair. I had a copy of armbian jammy on an sd-card, and this had booted OK before my stupid error. But with the SD card inserted, it stilll only booted to the busybox .
The trick was to start the boot, and then insert the SD-card mid-boot, and it then booted the working system from the SD-card. During the defective boot, there is a pause with a few lines of text on the screen before it then proceeds with more error messages and into the busybox. I inserted the SD-card during that pause and it completed the boot from the SD-card. (you may need to time this carefully, but it worked on first try for me)
Then you have a complete working system and can use armbian-config, > install system to eMMC and it copies the system to the eMMC and fixes the u-boot issue. It will now boot direct from the eMMC.
Took me a while to find this trick which I have not seen documented anywhere, but is essential to get a system running to effect a repair.