MongoDB Error: Failed to start mongod.service: Unit mongodb.service is masked

Reference:

reference: http://club.verimake.com/topics/36 http://stackoverflow.com/questions/37014186/running-mongodb-on-ubuntu-16-04-lts

on MongoDB: Failed to start mongod. Service: Unit MongoDB. Service is masked, you can follow the following steps:
opens the following file: 

sudo vim /etc/systemd/system/mongodb.service

Paste the following into the file and save 

[Unit]
Description=High-performance, schema-free document-oriented database
After=network.target

[Service]
User=mongodb
ExecStart=/usr/bin/mongod --quiet --config /etc/mongod.conf

[Install]
WantedBy=multi-user.target

execute the following command: 

sudo systemctl enable mongod.service
sudo systemctl daemon-reload
sudo service mongod start