Could you please share more detail about the Ambari server you are running.
1. Are you using any external server?
Ans. You need to check with Admin to provide you access or help resolve login issue.
2. Are you running Ambari server in your PC?
Ans: The default username and password is admin/admin. If you have setup and forget the credentials, please refer the following link to reset the admin password: https://hadooptips.wordpress.com/2015/12/18/resetting-ambari-admin-password/
3. Are you using Dezyre Lab Cluster?
Ans: Document to connect to Ambari is available on the dashboard under the course.
Hope this helps.