You may not be able to log into your paper trading account if:
Your live account has been migrated to a different legal entity. In that case, your old paper trading account was closed;
You requested a new paper trading account that isn't ready. It normally takes 24 hours to be activated;
Your paper trading account has been closed due to inactivity;
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).