The game instantly crashes everytime I try to use OptiFabric and Fabric API at the same time, and it doesn't give an option for a crash report so unfortunately I can't help out with that. It seems to work fine in 1.16.3 however so I'm not sure which mod is causing the issue.

I'm having this same issue

This should have (most likely) been fixed by 9f4a6fd, try updating to 1.8.16.