Before configuring a Cloud SQL instance to use private IP, you must have a VPC network at least one subnet in the same region as your Cloud SQL instances in place. From your 6 points list, it is not apparent how you configured this VPC network. You may refer to the "Configuring Private IP Connectivity" documentation
page for related detail. The page you refer to also states that private services access is implemented as a VPC peering connection between your VPC network and the Google services VPC network where your Cloud SQL instance resides.