When you attempt to remove an add-in, the dialog tells you that only an administrator can connect or disconnect the add-in:
This add-in is installed for all users on this computer and can only be connected or disconnected by an administrator.
Trying to remove an add-in that was installed using the option “All users” will result in this error message. If the add-in was installed for the current user this message will not appear.
When you install software, the screen may have an option to install it for the current user or for all users of the computer as is shown on this screenshot.
Most users click Next without paying much attention to this screen (using the default set by the developer), or set it for "All users" so the add-ins works for all Windows accounts.
Close Outlook and restart it using Run as Administrator then remove the add-in. Close Outlook and restart Outlook normally when finished.
To Run as Administrator, right-click on outlook.exe or the Outlook shortcut and choose Run as Administrator.
If Run as Administrator is not listed, hold Ctrl+Shift+Alt as you right-click on the shortcut to show the full menu.