Hey there, Bungie will not adjust anyone's cross-save settings on their end including deactivation. The only way to do it is through authentication. Rather than trying to authenticate Steam on the deactivation page, log out of Bungie.net and log in with Steam. If it logs you into this same Bungie Account then Steam will be automatically authenticated on the deactivation page. If it logs you into a different Bungie Account, asks you to make a new Bungie Account or link an existing one, it means it's a different Steam account than the one linked, hence why the authentication kept failing.