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You cannot mask services defined in /etc/systemd/system. It relies on creating a symlink there with the same name as the file which defines the service.

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From the big book of unhelpful error messages

# /bin/systemctl mask gitlab-runner.service
Failed to execute operation: Invalid argument
# systemctl status gitlab-runner.service
● gitlab-runner.service - GitLab Runner
   Loaded: loaded (/etc/systemd/system/gitlab-runner.service; enabled; vendor preset: disabled)
   Active: active (running) since Tue 2019-07-16 13:15:18 BST; 1h 54min ago
 Main PID: 3208 (gitlab-runner)
   CGroup: /system.slice/gitlab-runner.service
           └─3208 /usr/lib/gitlab-runner/gitlab-runner run --working-directory /home/gitlab-r...

The man page contents:

       mask NAME...
           Mask one or more unit files, as specified on the command line. 
           This will link these units to /dev/null, making it impossible 
           to start them. This is a stronger version of disable, since it 
           prohibits all kinds of activation of the unit, including
           enablement and manual activation. Use this option with care. 
           This honors the --runtime option to only mask temporarily 
           until the next reboot of the system. The --now option can be 
           used to ensure that the units are also stopped.