Oh, then I’m very sorry, but it doesn’t look good… 
You’re now probably here:
What you might try now:
check
fastboot flashing get_unlock_ability
I’m afraid it will show “0” (which means OEM unlock is disabled and that’s why unlock is not allowed…)
You can only enable “OEM unlock” if you can boot your system.
So did you already check all ways to boot your LineageOS installation?
I assume you already tried a “factory reset” and test if LineageOS boots then?
If you do not manage to enable “OEM unlock” then you’ll probably have to contact FP for a repair at Cordon repair service…