I have seen the old threads regarding this issue, and none of the proffered solutions seem to work for me.
I am an Org admin and attempting to install the Atlassian GitHub app into my Jira Cloud (Free) application. My only option, however, is to request the app. I can see the requests and whether I open the app directly from the request or search for it in the app marketplace -- my only option is to request (well, now I see "app was requested").