Prism Mac message: The document [name] could not be autosaved.

This alert from macOS happens that there is a problem with the project's data structures which prevents Prism from saving the project successfully.
There is no way to turn off autosave to prevent this message. 

Since the entire file is damaged and can't be saved, the fix is to try to save portions of the file. 

• Undo recent actions and try to save again.
• Delete recently created or modified sheet(s) and try to save again.
• Try to save parts of the project using File > Save Special > Save Family As...
• Force Prism to quit -- the problem file will effectively revert to the last successfully saved version."