I recently had to add a user for postgres in AWS RDS but it’s giving me this error:

ERROR:  must be member of role "test"

So here’s what I did:

First, create a user with CREATEDB capability:

CREATE USER testuser WITH CREATEDB PASSWORD 'testpassword';
\q

Connect again with this new user and create the database:

CREATE DATABASE testdb OWNER testuser;

That’s it.