How to set DBParameterGroup Family property for Postgres 10.6

You should set the Family property to postgres10.

Here is the list of available families for PostgreSQL:

You can find the list in the console when creating a Parameter Group. Alternatively you can find the list (with duplicates) using the following AWS CLI command:

aws rds describe-db-engine-versions --query "DBEngineVersions[].DBParameterGroupFamily"

This is because postgres10.6 is not an valid option available in Parameter group family section.
To get the list of all the available parameter group families, use the following AWS CLI command:

aws rds describe-db-engine-versions --query "DBEngineVersions[].DBParameterGroupFamily"

You can also check in AWS console by navigating through Parameter groups in AWS RDS and then in Parameter group family you will be listed with all the available group families in the dropdown menu.