By ensuring your internal file structure is flat and your naming conventions are exact, you can fix almost any conversion error.
If you have a folder of Scratch assets that you've zipped up and want to turn back into a functional Scratch file: Verify Contents : Ensure the project.json convert zip to sb3 fix
: Make sure all your Scratch assets (the project.json file, along with folders for costumes and sounds) are in the root of a folder. By ensuring your internal file structure is flat