Arf, oui, tu es en 16.04, il faut un PPA :
sudo add-apt-repository ppa:alexlarsson/flatpak
sudo apt update
sudo apt install flatpak
flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
flatpak install flathub com.skype.Client
On va y arriver ! 😉
