If the creator updates their mod to version 1.21.1, I will remove mine. For now, you can use the one provided in the link:

https://github.com/kierohere/JustEnoughResources/releases/tag/release

#514 happens on your fork

The mod builds just fine for 1.21 on Forge/NeoForge but has a runtime ClassNotFound issue. That is why I didn't release any version. See #514 (comment)