When using Amazon RDS, the user issuing the CREATE DATABASE must be a member of the role that will be the owner of the database. In my case, the superuser account I'm using is called root, and I'm going to create a role o which is going to own a database d:
postgres=> CREATE ROLE o;
CREATE ROLE
postgres=> CREATE DATABASE d OWNER = o;
ERROR: must be member of role "o"
postgres=> GRANT o TO root;
GRANT ROLE
postgres=> CREATE DATABASE d OWNER = o;
CREATE DATABASE
create role service_role with password 'some_password';
create database service_db with owner service_role;
ERROR: must be member of role "service_role"
grant admin_user service_role;
GRANT ROLE
create database service_db with owner service_role;
CREATE DATABASE