Wow... The package was removed from the Main Universe Ubuntu in favor of Snap. I just checked.
Canonical is really starting to fray my nerves with their High Pressure about Snap.
Here is the .deb package - you can download and just double click to run the installer.
https://github.com/flameshot-org/flameshot/releases/download/v12.1.0/flameshot-12.1.0-1.debian-10.amd64.deb