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