'no mountable file systems' error upon attempted installation of Debian 10.7.0 amd64 dmg

[absurdlapse <absurdlapse@protonmail.com>]

Hello,

I am trying to install Debian for the first time. I am on a 2020 iMac running macOS Catalina (which btw does not support the bash terminal anymore: uses zsh, so command line fixes might be different for mac than previously conceived). I've encountered an error ('no mountable file systems') when attempting to download Debian's image file on an amd64 architecture.

Things I've tried

1. Assuming the file was corrupted and downloading the disk image once more.
   
2. Rebooting my Mac.
   
3. Trying to mount the DMG on the command line in Terminal (I guess unrecognized syntax because the shell isn't bash anymore).
   
4. This one's what I really thought would fix it: Turning off secure boot. Unfortunately this didn't work either.
   

I'd appreciate any feedback from mac users if there are any.

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