The file permissions on the render directory may have gotten screwed up. Locate the render directory for the project and delete it. If you need what’s inside the directory then check the file permissions on the directory with the right click>get info.
The owner permissions should say that you can Read/Write if it doesn’t say that then the permissions need to be changed (not repaired, changed). If you are not an admin user on the machine then get the admin to sort it.
Emma