I hit this error after installing the plugin then going straight to setting up an org job. I probably should have read the setup instructions! I found (eventually) that I needed to add a Personal Access Token to the 'default' server that the plugin sets up for you, to allow it to use the GitLab API. This is done in the system configuration.
The error was simply "Owner 'xyz' is neither a user/group/subgroup" and didn't give any indication or hint that in fact it didn't have any credentials to use to access the API, and of course because the immediately preceding field to the group to scan field, in the job setup, is to select some credentials (these are for cloning), one naturally expects that it will use those credentials!