This error may be an issue with your system, rather than the DMG file you are downloading. Here is an article that covers a lot of different things you can check: macOS Error Opening .dmg Disk Image - No Mountable File Systems
This error may be an issue with your system, rather than the DMG file you are downloading. Here is an article that covers a lot of different things you can check: macOS Error Opening .dmg Disk Image - No Mountable File Systems