When you attempt to enable or disable an Office Add-In, the following error message may be displayed and the Add-In state does not change.
The connected state of Office Add-Ins registered in HKEY_LOCAL_MACHINE cannot be changed.
This happens if the Office application you’re using is running under standard privileges and hence cannot modify the Add-In startup state in the HKEY_LOCAL_MACHINE registry hive.
To resolve the problem, start the Office application elevated. To do so, right-click on the Office application shortcut and choose Run as Administrator. Then open the Add-Ins dialog to enable or disable the registered Office Add-In.
Using the Registry Editor
Alternately, you can change the Add-In startup state manually via the registry, by navigating to the Add-In registry location and modifying the LoadBehavior DWORD value manually.
Similarly, for Microsoft Excel:
To disable the Add-In, set the data for LoadBehavior to 2.
Third-party tool: OfficeIns (Office Add-ins Manager) from Nirsoft is a small utility that displays the details of all installed Microsoft Office (Word, Excel, Outlook, and so on…) add-ins on your computer, and allows you to disable/enable them.