I'm guessing if you hide and un-hide the on screen nav buttons it will show up.
Anyway Viber is a joke in Windows Phone,
let's hope they'll do some miracle and bring it up to par with the other major OSes in the not so distant future.
i experianced the same thing when i was trying out win10 TP on my Lumia 535.. apparently what i figured was its the scaling issue.. as if the win10 preview we get is not compatible perfectly with the phone and the viber app doesnt fit to the screen as well.. till u get the specific scaled version to ur phn model this will continue..