For people who encountered this error, I believe this is an error in Management Console. Before they fix this, you can create a read replica for a RDS instance in EC2 Classic properly with AWS command line.
aws rds create-db-instance-read-replica --profile xxxx --db-instance-identifier xxx --source-db-instance-identifier xxx --db-instance-class db.m3.medium --storage-type standard --no-multi-az --publicly-accessible --no-auto-minor-version-upgrade
The VpcId is the identity of the VPC to create the replica within. It is possible that the engine version you are using is not supported in EC2 Classic. See the documentation for determining this. You may have to move your RDS to a VPC.
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