I had the same issue last week when I created mine. Seemed like there was an issue with new players joining within the first few hours of the clan. I was able to accept the initial group request on the app, but the clan approval would give me the same message. And this was someone I gamed with often so I know the accounts were legit. In the morning, I logged in to the bungie website on pc. When I approved it on there it was fine. Now I can accept all requests on the app. Hope this helped, good luck!