Players using certain profile settings cannot have their Clan Join requests approved
Bungie is aware of and investigating an issue in which players with certain profile settings are encountering issues with their Clan Join requests. Players that have chosen to not display their linked game account on the Bungie.net profile may encounter issues accepting a Clan Join request. The Clan the player is attempting to join may also encounter issues approving their Clan Join request.
For more information, please see: LINKING AND UNLINKING DESTINY ACCOUNTS TO A BUNGIE.NET PROFILE