I used Simbrief before it was acquired by Navigraph. I even made a contribution to it. It appears that it is no longer usable without a paid subscription to Navigraph. I can find no way to access Simbrief through my Navigraph account. Too bad, it was good and useful.
You can most definitely log into Simbrief with an account without subscription. You can use the default Navigational Data which is typically about a year old.
Please try to log in and provide screenshots of any errors you experience using Guide to posting Screenshots.