You can try downloading the file in a different browser as well. If possible, try downloading the dmg again, turning off any download assistant plug-ins you may have. In most cases, the downloaded dmg file is actually corrupt or had an error downloading. If you see the 'no mountable file systems error' while opening a dmg, here's what you should try: The error was as the screenshot above shows trying to open a dmg (disk image), macOS showed the error 'no mountable file systems'.
I didn't find a lot of good search results addressing the issue, so I decided to write up a post about it myself. I ran into an interesting macOS error while working with a customer.
