First I have to recommend installing Roundcube directly from the packages published on the Roundcube site
https://roundcube.net/download/ rather then installing from the Ubuntu package repository since the Ubuntu package is Roundcube 0.9.5 which was released October 21, 2013 and is no longer supported. Often the packages also have other problems because they try to link to incorrect versions of Roundcube dependencies.
Now on to the problem your experiencing, do you have PHP installed?