Step 1: install flatpak:
sudo apt install flatpak
Step 2: install Viber:
flatpak --user remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo flatpak --user install flathub com.viber.Viber
That’s all, scan QR code with your phone and use Viber on Debian 10!
