For 26.3 (and likely above), 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, and was corrected back to this behavior in 26.3-Snapshot-5. 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 Traveler's Backpack on 26.3. If you're using this data pack on 26.3 and do not have Traveler's Backpack installed, you must remove the "travelersbackpack" 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:

advancednetherite					Advanced Netherite
travelersbackpack					Traveler's Backpack