Generally saving issues on Mac are permissions related. Easiest potential solution is to move the game to your applications folder. Otherwise you can try to manually adjust the permissions of the game's folder and/or save file to have both read and write permissions. If neither of those work, let me know. And have you played any other RPG Maker MV games on your Mac, and been able to save? That might narrow down if it's something with the game vs something with RPG Maker MV in general. Thanks