In Outlook a user can have special permission to access and manage another user's calendar. This permission is named delegated access and basically lets you perform tasks on behalf of another user in the other user's calendar.
When you working in a calendar on behalf of another user there is some functionality in Outlook that is no longer available and at the moment this includes not being able to use Modern Add-ins for Outlook 2013 and 2016. When working on behalf of others users Modern Add-ins will appear as grey icons where they usually are located when working in your own calendar and you will not be able to click and open the Modern Add-in.
The only way to use Outlook add-ins when working on behalf of another user is to use VSTO Add-ins. VSTO Add-ins is an older type of Outlook add-ins that are locally installed on the user's computer. There is no technical or visual difference between the AskCody VSTO Add-ins and the AskCody Modern Add-ins. It is simply a difference in how they are installed in the user's Outlook.
VSTO Add-ins are located in the Add-ins tab in Outlook and they will also work in the user's own calendar.