Can't start mongod from systemctl

Question

I've tried to install MongoDB 5.0 Community edition on Ubuntu 18.04 from the Official doc: https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/

Everything went fine until I tried to run mongod service:

$ systemctl start mongod
Failed to start mongod.service: Unit mongod.service is not loaded properly: Invalid argument.
See system logs and 'systemctl status mongod.service' for details.

I tried to gather infos:

$ systemctl status mongod.service
● mongod.service
   Loaded: error (Reason: Invalid argument)
   Active: inactive (dead)
$ sudo systemd-analyze verify mongod.service
mongod.service: Service lacks both ExecStart= and ExecStop= setting. Refusing.
mongod.service: Failed to create mongod.service/start: Unit mongod.service is not loaded properly: Argument invalide.
Attempted to remove disk file system, and we can't allow that.

If I try to enable the service instead, I have the following message:

$ systemctl enable mongod.service
The unit files have no installation config (WantedBy, RequiredBy, Also, Alias
settings in the [Install] section, and DefaultInstance for template units).
This means they are not meant to be enabled using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
   .wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
   a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
   D-Bus, udev, scripted systemctl call, ...).
4) In case of template units, the unit is meant to be enabled with some
   instance name specified.

Indeed, if I read the output from systemctl list-unit-files, the mongod service is flagged as static.

Here's my /lib/systemd/system/mongod.service contents (automatically written by installing mongodb):

  GNU nano 2.9.3                       /lib/systemd/system/mongod.service
[Unit]
Description=MongoDB Database Server
Documentation=https://docs.mongodb.org/manual
After=network.target
[Service]
User=mongodb
Group=mongodb
EnvironmentFile=-/etc/default/mongod
ExecStart=/usr/bin/mongod --config /etc/mongod.conf
PIDFile=/var/run/mongodb/mongod.pid
file size
LimitFSIZE=infinity
cpu time
LimitCPU=infinity
virtual memory size
LimitAS=infinity
open files
LimitNOFILE=64000
processes/threads
LimitNPROC=64000
locked memory
LimitMEMLOCK=infinity
total threads (user+kernel)
TasksMax=infinity
TasksAccounting=false
Recommended limits for for mongod as specified in
http://docs.mongodb.org/manual/reference/ulimit/#recommended-settings
[Install]
WantedBy=multi-user.target

The /var/log/mongodb/ folder is empty though.

Note that I had a previous versions of MongoDB installed, so I ran through the steps on this post:Apt-get fails on 16.04 or 18.04 installing mongodb

And I tried to installed MongoDB 4.2 after that. And the issue persists.

It is indeed installed though, as I can see its version:

db version v4.2.18
git version: f65ce5e25c0b26a00d091a4d24eec1a8b3a4c016
OpenSSL version: OpenSSL 1.1.1  11 Sep 2018
allocator: tcmalloc
modules: none
build environment:
    distmod: ubuntu1804
    distarch: x86_64
    target_arch: x86_64

What do these systemctl errors mean? What should I try to do? Thank you for reading.

EDIT: Note that I need to run the mongod command through sudo to have it working from the terminal.

So I tested to modify the mongod.service file in two waYS:

• First with User=root, Group=root
• Then by adding mongodb to my sudo group and using:

User=mongodb
Group=mongodb
EnvironmentFile=-/etc/default/mongod
ExecStart=sudo /usr/bin/mongod --config /etc/mongod.conf

Both resulted in the same error: Invalid argument