Are you sending or receiving the clan invite? Is the invitation through the game or through the Call of Duty app? We are aware that some players are experiencing functionality issues and Beachhead is working on it, you can find more updates here Beachhead Studio. In the mean time, please try the following:
1. Reinstall the Call of Duty app
2. Relog into the Call of Duty app
3. Relink your game account to your Call of Duty account
Please let us know if the the above resolved your issue.