Read carefully and check the structure of the archives (zips) or folders (directories), make sure there are no additional ones.
Wrong: pack.zip → pack (folder) → pack.mcmeta pack (folder) → pack (folder) → pack.mcmeta
Correct: pack.zip → pack.mcmeta pack (folder) → pack.mcmeta
The world cannot be zipped inside the saves folder.
Wrong: world.zip → level.dat world.zip → world (directory) → level.dat
Correct: world (directory) → level.dat