Hi, this is an issue in Ubuntu that occurs with the Launcher bar.
Put simply, the launcher buttons are "stealing" the mouse over events away from EotU. Setting the launcher to auto-hide (Appearance->Behaviour->Auto-hide the Launcher set to ON) will help mitigate the problem for you. Alternatively, you can play the game in Windowed mode with a lower resolution than the fill screen and, assuming the Window doesn't overlap the Launcher, you'll be able to click the buttons.
I have already contacted Epic about this issue with the Unreal Engine 4, but I'm yet to hear if they're working on a fix.