There are 3 possible issues for this
1. The .sb3 file is corrupted
It is hard to tell if the .sb3 file is corrupted just by looking at it, but if you made any edits to the file on purpose or by accident, then that would be the reason why it doesn't work. To solve this, if you can access the .sb3 file on scratch, download it and re-upload it, being careful not to edit it.
2. The file is too large
This is probably not the case, but if it is, there is no easy solution. The best way would be to go to https://garbomuffin.github.io/sb3o/ and upload your file. This reduces the size by about 20%. After you finish this, re-upload it to github and see if it works.
3. TurboWarp doesn't support GitHub file reading
When I visit the GitHub URL, it says that the file is too big to display the raw data. If GitHub can't display it, maybe TurboWarp can't read it. The solution to this would be to just switch to another hosting platform if possible.