Version 0.1.4 on Modrinth is listed to have support for 1.21.x, however will not launch with 1.21.1 as the mod is expecting exactly version 1.21.

If you ever run into a mod that should work on a certain version, but hard depends on a minecraft version; then you can just do this:

Create a file called fabric_loader_dependencies.json and put it into your config folder.
Then add this to the file you just created.

{
	"version": 1,
	  "overrides": {
		"nbttooltips": {
			"-depends": {
				"minecraft": "1.21.1"
			}
		}
	}
}

Thank you, I didn't know I could do that!