This problem seems to occur most often with a Kwikset lock which has a different “fingerprint” than SmartThings expects. It still gets added to your account, but it gets added as a lock without a keypad, so then SLGA (Smart Lock Guest Access) doesn’t know how to set the lock codes for it.
The following is a very helpful post from @mlchelp that explains in detail how to fix this problem.
Kwikset zigbee lock won’t work (March 2023) - #24 by mlchelp