Unrecognised Service for existing, working service - Ubuntu 18.04

Solution 1:

Use the real command, which for systemd would be:

sudo systemctl enable my-service

For reference, you do not have to use the main system directory for the unit file if you want it to run for a normal user, e.g. your own. You can create it in a directory such as:

~/.config/systemd/user/

Then you can do:

systemctl --user enable my-service

This avoids the need for sudo

As you have noted in your comment, when running per-user instances of systemd the User directive should be absent

Solution 2:

Note:
In other instances that you get the same error:
The service command puts the object first !

service foo start

The systemctl command does the opposite:

systemctl start foo

Also, in service, you cannot add ".service", while you can if you use systemctl.