This mod requires Fabric loader 0.16.10 in the mod json file which prevents loading with anything but the very latest version of Fabric loader, this should target the minimum supported version not the latest available version.

Also have this issue

1.21.5 Fabric API requires the latest Fabric Loader, so I don't have a lot of room to downgrade. I'll see what I can do in the next OptiGUI 3 alpha.
Is downloading the latest Fabric Loader a problem?