@Regis Previously open source, the new versions of ambari (2.7.5) and HDP (3.1.5) have moved behind a paywall in an open core strategy. You cannot access them without cloudera subscription.
I recommend to use the last free versions: Ambari 2.7.4 and HDP 3.1.4.
You can find these repos here:
https://docs.cloudera.com/HDPDocuments/Ambari-2.7.4.0/bk_ambari-installation/content/ambari_repositories.html
https://docs.cloudera.com/HDPDocuments/Ambari-2.7.4.0/bk_ambari-installation/content/hdp_314_repositories.html
Also below is more information about the Authorization needed for Pay Walled Repos:
https://docs.cloudera.com/HDPDocuments/Ambari-2.7.5.0/bk_ambari-installation/content/access_ambari_paywall.html
If this answer resolves your issue or allows you to move forward, please choose to ACCEPT this solution and close this topic. If you have further dialogue on this topic please comment here or feel free to private message me. If you have new questions related to your Use Case please create separate topic and feel free to tag me in your post.
Thanks,
Steven @ DFHZ