I've recently noticed a lot of people are creating topics in the Bugs and Glitches forum saying that their project cannot be saved, while I was unable to reproduce the bug and assume that this is an issue on the project's side. This topic exists to explain why this happens, and how to get around it.

Sounds, Costumes or Code with Large Sizes
This is probably the most common cause because a lot of people download very long music tracks for use in their project.

Scratch has a size limit of 5 MB (megabytes) for each sound and costume. This means many high-quality or long sounds downloaded from the internet may be too large to upload to the Scratch website. The data forming the project (the project.json file) must also not exceed 10 MB.

To check if your project has any large files, click the File dropdown and click “Save to your computer”. Rename the file from a .sb3 file to a .zip file, then open the zip file. Check if any files other than project.json inside the zip file exceed 5 MB, and if project.json exceeds 10 MB. If so, this is the cause of your project not saving.

Internet Connection Issues
Sometimes your internet connection may be unable to upload your file to Scratch. Consider using much smaller files and check if the project can be saved.

You may have also unexpectedly been disconnected. Try connecting again if this is the case.

Session Expiry
Scratch sessions last up to 2 weeks (this can be reset by e.g. creating a forum post). Your session may have expired while you were creating the project. If this is the case, open Scratch in a new tab and log back in. You should then be able to save your project.

Server Issues
Sometimes, the Scratch servers may be having an issue preventing projects from being saved. If this is the case, download your project, and when the issue is solved go to your project's code then File > Load from your computer. Then select your project's file and save the project.

Thanks! it took me a few minutes to realise it was the music from caseoh's basics that was too big (ofc) The game is here: https://scratch-mit-edu.ezproxyberklee.flo.org/projects/1136653876/