Ditto. Fortunately I never had the problem issue of erasing the DBX/etc signatures, thanks to those who reported it here, but before setting up Secure Boot on my FW-12, 13th Gen (running NixOS, via Lanzaboote), I had already updated my firmware per:
Unfortnately, that decision landed me in more or less the same situation as other folks here. Sure enough, though, by going back into BIOS, enabling “Restore Secure Boot to Factory Settings” then going through the process again as @benjarobin reports above, I was able to get secure boot set up, if only at the cost of rolling back my firmware. 
Hopefully some kind of fix will be forthcoming, if not already available? 