@M R
As you are using your own "Postgres" instance and we see that it is running fine as well. So i am suspecting that you might not have created the Ambari Database inside the Postgres which is needed if we want to use Non Default Database with Ambari.
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Please refer to the following doc to verify if you have created the ambari database schema and users properly or not?
https://docs.hortonworks.com/HDPDocuments/Ambari-2.5.1.0/bk_ambari-administration/content/using_amba...
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If you have followed the doc then you should be able to connect to ambari DB on your postgres as following:
# psql -U ambari ambari
Password for user ambari: bigdata
psql (9.5.5)
Type "help" for help.
ambari=>
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Also please check the "ambari.properties" JDBC properties are correct and desired or not?
# grep 'jdbc' /etc/ambari-server/conf/ambari.properties
server.jdbc.hostname=standaloneambari.example.com
server.jdbc.driver=org.postgresql.Driver
server.jdbc.rca.user.name=ambari
server.jdbc.user.passwd=/etc/ambari-server/conf/password.dat
server.jdbc.database=postgres
server.jdbc.database_name=ambari
server.jdbc.rca.url=jdbc:postgresql://standaloneambari.example.com:5432/ambari
server.jdbc.rca.driver=org.postgresql.Driver
server.jdbc.port=5432
server.jdbc.rca.user.passwd=/etc/ambari-server/conf/password.dat
server.jdbc.user.name=ambari
server.jdbc.postgres.schema=ambari
server.jdbc.url=jdbc:postgresql://standaloneambari.example.com:5432/ambari
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Above will show the configured Postgres DB hostname & Port and ambari Schema .... those should be available and accessible. Password can be seen in the following file for verification:
# cat /etc/ambari-server/conf/password.dat
bigdata
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