Please log out of your Bungie.net profile here on Bungie.net, then log in using the account that you have set as your main cross-save. You should then be able to adjust your clan stuff. You should be able to then send the request to the clan you're trying to join. Since cross-save enabled accounts can only have one active clan, and the active clan is tied to the active account, and an account can only tinker with clans on Bungie.net while that account is logged in, then you need to log in with your active cross-save platform account here on Bungie.net to tinker with clans.