PolyUI has drastically changed this was first made; please use OneConfig directly instead of implementing PolyUI in Minecraft yourself.
an example mod showing how to implement PolyUI into a modern Minecraft environment.
This should work for all versions 1.13+, and it is currently implemented for 1.20.1-fabric.
The default implementation depends on fabric-api-commands-v2 so the UI can be opened with the /examplemod command.
With some minor versioning tweaks, this can work in a full multiversion setup.