What is happening if my guess is right is you are trying to use your community username/password this will definitely fail.
Ambari 2.7.x is available for companies with valid HDP 3.x support licenses you have an active subscription with Cloudera as you can see below access is denies as I used my community login.
Here is the HDP support Matrix
Starting with the HDP 3.1.5 release, access to HDP repositories requires authentication. To access the binaries, you must first have the required authentication credentials (username and password).
Read accessing HDP repositories
Hope that helps