In Order for the connector to link to the QuickBooks desktop datafile, the user needs to have accepted a QuickBooks Application Certificate.
Typically this will pop up as a prompt the first time the user downloads the connector and looks like this -
The user must select "Yes, always; allow access even if QuickBooks is not running", as this will allow the Connector to communicate with the QuickBooks file even when the users QuickBooks session is not active.
To get back to the certificate settings within QuickBooks go to:
Edit(1) > Preferences(2) > Integrated Applications(3) > Company Preferences(4)
You will then need to select the application named 'QuickBooks Connector' and click remove (5).
Once you have removed the certificate, you can then re-run the installation process for the connector - either by running the downloaded (this might be in the downloads folder and might be named <your company>-quickbooksdesktop-connector) or alternatively by re-visiting the connector download page. During the installation process you will then receive a prompt asking for permission from the application certificate.