Q:

Failed to start mongod.service: Unit mongod.service not found

I follow all the steps mention in MongoDB installation documents for Ubuntu 16.04.
Steps 1: 
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2930ADAE8CAF5059EE73BB4B58712A2291FA4AD5

Steps 2: 
echo "deb [ arch=amd64,arm64 ] https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.6.list

Steps 3: 
sudo apt-get update

Steps 4: 
sudo apt-get install -y mongodb-org

Steps 5: 
sudo service mongod start

when I started MongoDB got an error as: 

Failed to start mongod.service: Unit mongod.service not found.

A:

Most probably unit mongodb.service is masked. Use following command to unmask it.
sudo systemctl unmask mongod
and re-run
sudo service mongod start

A:

Please follow the below steps, it should work.
1 - Uninstall current installation completely
Source - official instructions
sudo service mongod stop

Remove Packages
sudo apt-get purge mongodb-org*

Remove the folders
sudo rm -r /var/log/mongodb
sudo rm -r /var/lib/mongodb

2 - Reinstall as described on official site, I will just write down the all steps. enter link description here
Import the public key
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2930ADAE8CAF5059EE73BB4B58712A2291FA4AD5

Create a list file for Ubuntu 16.04
echo "deb [ arch=amd64,arm64 ] https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.6.list

update the list
sudo apt-get update

Install the latest package
sudo apt-get install -y mongodb-org

3 - Now it should work, please try below command
sudo service mongod start

and check the status
mongo

it should appear the mongo shell

A:

Failed to start mongod.service: Unit mongod.service not found
If you are following the official doc and are coming across with the error above that means mongod.service is not enabled yet on you machine (I am talking about Ubuntu 16.04). You need to do that using following command
sudo systemctl enable mongod.service

Now you can start mongodb using the following command
sudo service mongod start

