Error message: "You cannot save it under its current name. Choose Save As and save under a different name."

This message from Prism Mac is misleading. In fact, the problem is likely to be that the folder you have selected is write-protected, so Prism is not allowed to save a file into it.  It won't help to choose a new file name. You'll need to choose a new location. This may be a sign of larger problems with your disk or file system.