It's been a while here,
But adding my 2 cents for anyone encountering this issue still.

I've been using ACK to create an Aurora-Postgresql DBCluster and attach a DBInstance to it, and I've been getting a very similar error, but for the port setting ("Set database endpoint port number for the DB Cluster."). This led me to think the issue is in the DBCluster spec, but it is actually an issue with the DBInstance spec.

If you have specifications that need to be defined on a cluster level, defining them at the instance level as well will cause this error. So you need to remove the VPC setting (in my case, the port) and other parameters from the DBInstance applied configurations. That's what resolved the issue for me.