Do you have the Call of Duty app?
If yes, follow this:
1. Delete Call of Duty App
2. Reboot your Phone/Device
4. download the Call of Duty® Mobile
5. Play a few games before logging-in
6. Log-in to app
Tell me if this helps. Well, if you don't have the Call of Duty app, just follow Step 3 then reboot your console and then try joining a Clan.
In the Call of Duty app, when you apply to a clan, you only need to press the helmet option once. At this time, it doesn't give you a confirmation that your application was sent, so pressing it again will trigger that error message. Rest assured you application has gone through, and at that point is up to the discretion of the Clan Leader to accept your invite.