Ensure your ZIP archive does not contain a main folder with the files inside it. The project.json file must be immediately visible when the archive is opened.
Standard ZIP files often contain nested folders (e.g., /costumes/ , /sounds/ ). An extra-quality SB3 conversion requires all assets, alongside the project.json file, to sit at the absolute root level of the archive. 3. Strict UTF-8 Encoding zip to sb3 extra quality