Ed, from my experience with serial/tty devices on linux, when I run into an issue with a port like this, usually it is a permissions issue.
On many modern linux systems, you need to make sure your user account is in the group that "owns" the tty device, usually the "dialout" group. You can find guides online on how to add users to groups in linux mint.
Another possible issue is the flatpak release itself, sometimes flatpak versions of apps are missing the correct permissions to access ports/devices on your system. You can install a program called "flatseal" from the linux mint app store (icon is a roll of tape) to edit permissions of flatpak apps installed on your system, you can use it to give your copy of chirp extra permissions to access more apps/files/devices.
Not certain that either of these will work for you, but good luck!
- Dev, KZ1Y