When you registered your CBT Key you were asked to enter your first name and last name.  This information becomes your login ID and must be entered EXACTLY the same way as registered or you will get the "invalid credentials" message.
If you put spaces, titles or certifications after your name, then you must use those same characters to log in.  For example, if you registered as FIRST: John and LAST: Doe, RN then every time you access the program you must enter your last name as Doe, RN.
Similarly, if you normally go by a nickname (Christy) but you registered the CBT program under the name Christina, then you will need to log in as Christina.
If you don't remember the name you used, you can contact the Training Department for assistance.