"Create Aurora Read Replica" option not available to migrate from RDS Postgres 11.12 to Aurora 11.12.

Looks like an issue with the console. Can you please try using AWS CLI for that:

To create Aurora replica cluster use the following CLI command "create-db-cluster" with "--replication-source-identifier" option first and then add instances to it using "create-db-instance".

Here is an example:

$ aws rds create-db-cluster --db-cluster-identifier "name of new replica cluster" --engine aurora-postgresql --db-subnet-group-name "subnet-group-name-here" --vpc-security-group-ids "security-group-id-here" --replication-source-identifier " postgreSQL RDS source-instance-ARN" --engine-version 11.12 --storage-encrypted --kms-key-id "KMS-key-ARN-here"

Note-

** Above command is a one liner**

** If your source RDS instance has storage encryption, you may need to also include the "--storage-encrypted" flag to the above command**

** The option "--replication-source-identifier" is mandatory to setup replication between the resources**

At this point Aurora replica cluster should be created with no instances. To add instances to the cluster you can use "create-db-instance". Below is the example:

$ aws rds create-db-instance --db-cluster-identifier "name of new replica cluster above" --db-instance-identifier "new aurora instance name" --db-instance-class db.t3.medium --engine aurora-postgresql