The only possible reason I might have based on the information that you currently provided is that maybe you were not logged in to your account, but in fact logged in as a guest. Therefore as a guest you cannot join a faction, so try logging in to your account and trying it. It might also be helpful to try to join a different faction to see if it is just a problem with the faction you are trying to join. You can always withdraw your application letter if needed. Could you please provide a bit more information as to the process you took in joining a faction, which one and exactly when the error occurred (a screenshot would be great too). If all else fails then just go onto IRC and join the room #swc-help. There you can get real time help if noone is available to help you here or if you are just too impatient.