But still had the same error. I rebooted and had the install button (with the other error Martin noticed too on #3). Unfortunately that doesn't worked too. I had an other error (the third...) that I attached. Note the installation work from command line.
I had the same error. So I installed gnome-software- plugin- flatpak and flatpak :
$ sudo add-apt-repository ppa:alexlarsson /flatpak && sudo apt update
$ sudo apt install flatpak
But still had the same error. I rebooted and had the install button (with the other error Martin noticed too on #3). Unfortunately that doesn't worked too. I had an other error (the third...) that I attached. Note the installation work from command line.