error: Unit mongodb.service is masked when starting mongodb
Solution 1:
Unit mongodb.service is masked.
So unmask it:
sudo systemctl unmask mongodb
Why are some systemd services in the "masked" state? explains masking.
Solution 2:
With 3+ version of mongodb. You dont have to start mongodb but you just have to start mongod. So proper way to start mongod will be
sudo systemctl start mongod