If, when running AutoPkg, you get an error like

failed: hdiutil: attach failed - no mountable file systems

but you’re able to mount the .dmg manually (i.e., it’s not a corrupted download), double-check you don’t have a restrictions profile installed that requires you to authenticate when mounting disk images. That setting is pretty much useless anyway if you’re an admin user (you won’t be prompted in the GUI to authenticate when mounting disk images via Finder), and it will just make it so when AutoPkg is trying to run

/usr/bin/hdiutil attach -plist -mountrandom /private/tmp -nobrowse /PATH/TO/DOWNLOADED/DISKIMAGENAME.dmg

that it will choke with that error.

(Oddly enough, if you just do hdiutil attach without all those other options, it will still work just fine.)

Special shoutout to @elios on the MacAdmins Slack for helping another user figure this out three years ago, and so that’s how I was able to solve my problem.