Adding new user in Postgres
You must grant a user:


CONNECT on the database (if public doesn't have it by default)
USAGE on any schemas like the public schema (again, if the public role doesn't have it by default)
... and the rights you want on a given table.


I suspect you (or RDS) have revoked CONNECT on the database and/or USAGE on the schema(s) you're using, so the role doesn't have the rights to get to the point where it can use the table privileges.