The current (v3.4) installation manual for MongoDB on Ubuntu:

https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/

Is missing a step to create systemd file for Ubuntu 16.04, as listed on MongoDB v3.2 manual.

https://docs.mongodb.com/v3.2/tutorial/install-mongodb-on-ubuntu/#ubuntu-16-04-only-create-systemd-service-file

This step is necessary in Ubuntu 16.04 Xenial to be performed before users be able to use `service mongod`.

https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/#start-mongodb