I have been using Flamshot for many years on Cinnamon installed on Ubuntu (not using Mint)
When I decided to move apps to use flatpak Flamshot fails to start:
$ flatpak run org.flameshot.Flameshot
QIBusPlatformInputContext: invalid portal bus.
Gtk-Message: 12:40:46.046: Failed to load module "xapp-gtk3-module"
Gtk-Message: 12:40:46.046: Failed to load module "xapp-gtk3-module"
(flameshot:2): GLib-GIO-ERROR **: 12:40:46.126: Settings schema 'org.cinnamon.desktop.interface' is not installed
* Flamshot installed from `apt` works fine under Cinnamon.
**System info**
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 21.10
Release: 21.10
Codename: impish
$ cinnamon --version
Cinnamon 4.8.6
$ flatpak list | grep flameshot
Flameshot org.flameshot.Flameshot 11.0.0 stable system
The missing component does not see to be available:
$ flatpak search org.cinnamon.desktop.interface
No matches found
Problem
(flameshot:2): GLib-GIO-ERROR **: 12:40:46.126: Settings schema 'org.cinnamon.desktop.interface' is not installed
$ flatpak search org.cinnamon.desktop.interface No matches found