In a surprise move, Ubuntu developers have agreed to stop shipping Flatpak, preinstalled Flatpak apps, and any plugins needed to install Flatpak apps through a GUI software tool in the default package set across all eight of Ubuntu’s official flavors, as of the upcoming Ubuntu 23.04 release.
The instance needs to have https://github.com/LemmyNet/lemmy-ansible/pull/114 applied, it might not be in the version you are running yet.