Whenever I try to play in singleplayer with the fabric API mod my game crashes with an error message of:
The game crashed whilst exception in server tick loop
Error: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
The version of Minecraft that I am playing on is fabric-loader-0.8.8+build.202-1.15.2 of Fabric and the only mod I have that requires the fabric API is ReplayMod.
Thank you for looking into it at some point.

We are going to need a full log to figure out the issue. Please attach that if you can.

If what you are wanting is the full crash report then here it is.
Crash Log.txt

If needed for some more reference points, in some further testing, I found the same Issue in the Fabric API files named;
fabric-api-0.16.0+build.321-1.15.jar and
fabric-api-1.16.2+build.322-1.15.jar

All of the versions of Fabric API for 1.16.1 are working fine no crashing.

You're using a version of Fabric API for 1.15.2 in that log. Please update to the latest version of Fabric API for 1.16.1

I have and it has been working perfectly.

Closing since issue seems to have been resolved.