Installing packages by apt-get is not possible because Ubuntu touch devices have a readonly filesystem by default.
It can be disabled through USB:
Just enable the developer mode on your device,install the fablet-tools on your desktop and run. 'phablet-config writable-image'.
But be aware, OTA-Updates will be disabled and your device might crash, because this is not the usual way of developing applications for Ubuntu touch. You should do this just for experimental usage.
Further information can be found here underthe topic 'enablethe read-write mode: https://developer.ubuntu.com/en/start/ubuntu-for-devices/installing-ubuntu-for-devices/