Since some other mods support 1.21.1 but not 1.21, I am still using the 1.21 client.
I tried to modify the fabric.mod.json in your NBTac-FABRIC-1.21.1-1.3.9.jar to make the mod support 1.21, but it failed.
If you have time, can you make a 1.21 version of NBTac-FABRIC-1.3.9?

Ok, I'll soon release 1.21 version