failed in start mongod on ubuntu code example

Example 1: Failed to enable unit: Unit file mongod.service does not exist.

> systemctl list-unit-files --type=service
...
mongodb.service             disabled
...

Example 2: Failed to enable unit: Unit file mongod.service does not exist.

> cd /lib/systemd/system
> sudo touch mongodb.service
> sudo nano mongodb.service
[Unit]
Description=An object/document-oriented database
Documentation=man:mongod(1)
After=network.target

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

[Install]
WantedBy=multi-user.target