The BJCP uses several web sites, all with different credentials. So the first thing to do is make sure you have access to the specific area you want to use. The online testing site is administered by a third-party company, and uses their own credentials. Once you are a judge (and have a BJCP ID and password), you can access the member portal that contains your judge record and other services. The web site also has a forum that requires a different login. So make sure you are accessing the correct site and that you have valid credentials.
If you haven’t taken a judging exam, you will not have a BJCP ID and password. Your online exam certificate ID is not a Judge ID and will not give you access to the main web site.