Now that ambari has come back to life, this conversation is relevant despite other stack members feels. Additionally 100s if not 1000s of companies are still using free open source ambari w/ free open source HDP.
When you are building ambari 2.7.5 from source, you need to feed it a stack. HDP was one of them, that is no NLA unless you copied it.
The last free HDP repos you need are found here:
https://www.makeopensourcegreatagain.com/rpms/
And you can find instructions about how to use these here:
https://github.com/steven-matison/dfhz_ddp_mpack