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

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

Most probably unit mongodb.service is masked. Use following command to unmask it.

sudo systemctl unmask mongod

and re-run

sudo service mongod start

You are missing a 'b' I think?

sudo service mongod start

should be

sudo service mongodb start

I think this is the case?