I diligently followed the instructions at https://www.cyanogenmods.org/how-to-download-and-install-official-lineageos-16-for-oneplus-6/ to upgrade my phone to Lineage. I was concerned about the command “fastboot boot twrp-3.2.1-0-enchilada.img” . Other sites suggest using commands like “fastboot flash recovery twrp.img”. I am aware of the difference between Boot and Recovery, but assumed the enchilada version includes boot code. I was wary about writing to the Boot partition, but decided to follow the instructions.
However, my phone is now locked up, displaying a message “the boot loader is unlocked and software integrity cannot be guaranteed”. It seems therefore that fastboot has corrupted the boot code and hence crashed the phone. I found that if I hold Volume Up + Volume Down + Power for several seconds the phone will reboot. If I continue holding them it opens Fastboot. From my PC I can run Fastboot to again flash boot, but the end result is the same. I have tried to flash Recovery, but it fails, saying no recovery_a partition.
When the phone is in Fastboot I can run Tool All In One on my PC and from it I can open Recovery Flasher and Device Rooter. From this I can install TWRP 3.3.1 Q OP6T and it opens on the phone. From TWRP I can reboot Bootloader, but if I try to reboot Recovery I get the error message described above.