Unable to add addin's in Outlook

When attempting to add an add-in in Outlook, clicking on "Get Add-in" redirects us to a web browser. From there, if we select any add-in and click "Get," we are taken to an O365 admin center page where we receive a "Permission Denied" message. However, when attempting to add an add-in in Excel or Word, clicking on "Get Add-in" opens a small window where we can easily add and immediately use any desired add-in.

It is important to note that we are currently utilizing the GCC G3 license.