Multipass Source path does not exist

I'm running multipass on Ubuntu Desktop 21.04 to create many Ubuntu VMs on this Ubuntu machine. I need to mount a folder I created at the root directory /app to one of the VMs. While ls -l /app shows the folder, multipass mount /app newvm gives the error that Source path "/app" does not exist.

If I try to use autocomplete with multipass mount / and press tab it shows most of the folders in my root directory but not the /app folder.

For testing, I created a new root level folder using sudo mkdir /bob. Then multipass mount /bob newvm. Still same error. Source path "/bob" does not exist

Why would multipass not show all the root level folders?

If Multipass was installed the 'default' way snap install multipass, it won't be able to access directories outside of your /home, /mnt, or /media because of its strict confinement policy. Classic snaps, installs using the --classic parameter, don't have this restriction and have much greater access to the system as they aren't tightly sandboxed.

A little more on confinement policies.

If it's not convenient to move the /app directory to /mnt or /media (and I assume its not convenient to put it in your /home), you can try installing multipass with snap install multipass --classic

Interesting to note the video on the snap store page uses the --classic flag in the demo, but the install button does not include it.