First up, the current version of pi-hole will not work on Trusty. Ubuntu 14.04.5 is not on the list of officially supported distros.
That said, if you would like to try an older version of pi-hole, you can change to version 3.2.1, which may well work on your system. (I have personally tested downgrading from the current Pi-hole to version 3.2.1 on ubuntu 14.04.5 by this method, and it is working at this time).
If you have not already, please ensure that your system software up to date. Either by the Software Updater, or from the commandline:
sudo apt update && sudo apt upgrade
After rebooting, the instructions to change the version of pi-hole are here
(Also note the additional instructions from 2018-02-07 at the bottom of the first post - edit the file /etc/dnsmasq.d/01-pihole.conf as directed before you follow the main instructions).