As part of 1.21, Mojang changed the directories for various tags to use singular names instead of plural.

Your mod uses tags in one or more of the old directories that was changed! This means those tag files will no longer load in 1.21 unless you update the directories!

The affected directories and their new counterparts are:

• tags/blocks -> tags/block
• tags/items -> tags/item
• tags/entity_types -> tags/entity_type
• tags/fluids -> tags/fluid
• tags/game_events -> tags/game_event

Please ensure that any of these that you are using is updated to the new name in all namespaces (including your own, minecraft, and c).

Oh wow thanks for letting me know. I don't know why I didn't notice, because all of my tag translations are using singular form there.
I've updated the mod now with these changes. Thanks again for notifying me about it!