Note: Fabric API version 0.126.0+1.21.6 is not fully compatible with the Wurst build for 1.21.6-pre3. The game should still launch, but you won't be able to use X-Ray with this version.
This will be fixed whenever pre4 / rc1 comes out, but for now please use Fabric API 0.125.3+1.21.6 instead. (That's the version listed on the Wurst download page.)
Edit: Nevermind. It seems like I misunderstood the Fabric API changelog.