There are several zip to SB3 converters available online. Choose a reputable converter that suits your needs.
Cause: The project.json is missing or corrupt.
Fix: Open the zip folder. Is there a project.json at the root level? If it is inside a subfolder (e.g., /src/project.json), Scratch cannot read it. Move the JSON to the root of the zip before renaming. Zip To Sb3 Converter
If you need to study or automate this conversion: There are several zip to SB3 converters available online