For 26.3+, you cannot have unknown data in a data pack or else Minecraft will refuse to reload any loaded packs. This is classified as intended game behavior. As such, you will have to remove anything in this folder that doesn't relate to a mod you've installed.

For example, this pack supports Biomes O' Plenty on 26.3. If you're using this data pack on 26.3 and do NOT have Biomes O' Plenty installed, you must remove the "biomesoplenty" folder from this folder.

If you're playing vanilla, remove everything in this folder EXCEPT "minecraft".

Here's a list of the folder names and what the mod names are for reference:

alexsmobs					Alex's Mobs
biomesoplenty				Biomes O' Plenty
ecologics					Ecologics
minecraft					Minecraft (the game itself, I hope this is obvious)
the_sift					Mielon's The Sift