Failed to start mongod.service: Unit mongod.service not found on Ubuntu 20.04

i followed the official instruction of mongodb install_intruction on my Ubuntu 20.04 LTS,
when i started mongodb by this line:
sudo systemctl start mongod
i got an error like this:

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

of course i tried to reload as mongdb instruction:

sudo systemctl daemon-reload

it still shows:

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

then i tried:

systemctl list-unit-files --type=service

to find mongod.service but i couldnt find it. and i tried:

ls /lib/systemd/system

that didnt work either there is no mongod.service file

i really need a hand!!!