Installing the Unifi controller on Ubuntu can be quite a mission if Mongodb isn't installed. Install Mongodb with:

sudo apt install mongodb

There are number of issues around Plymouth which result in the Unifi controller not being able to start. A simple solution is just to kill Plymouth with:

sudo pkill plymouth

To view the status of the Unifi controller execute:

sudo systemctl status unifi

You should see a green dot showing the controller is running. If you don't see this you can run:

sudo systemctl restart unifi