Looks like the issue is that the UniFi server can’t be installed with a version of MongoDB newer then 3.6.0

https://help.ubnt.com/hc/en-us/articles/220066768-UniFi-How-to-Install-Update-via-APT-on-Debian-or-Ubuntu

Fix the issue by installing an older version of MongoDB

wget -qO - https://www.mongodb.org/static/pgp/server-3.4.asc | sudo apt-key add -
echo "deb https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.4.list
sudo apt-get update

Then install MongoDB with

apt install mongodb-org-server

You may run into issues if you already have a version of Mongo installed. Uninstall it and then rerun the above install command.