The only thing that I can think of - did you install Viber after installing Google Play Services? Because if you install the app first and then Google Play Services, the app won't work.
Maybe you can try to uninstall Viber, then uninstall Google Play Services, then install Google Play Services back, open Google Play store so that it will finalize the settings (you don't have to login to store) and then install Viber and try again?
Make sure Viber has phone permissions.
I just installed it as normal and it worked immediately, without any issues so I am assuming its something to do with Google Play services config.