You may not be able to log into your paper trading account if: You requested a new paper trading account that isn't ready. It normally takes 24 hours to be activated.
Your live account is closed. In that case, your paper trading account will also be closed.
You have an older account and have never activated a paper account (newer accounts include a paper account).
See also: What is the username of my paper account? and How can I reopen a closed paper trading account?