PostgreSQL_role.md

$ psql -d postgres
postgres=# create role "app_name" login createdb;
postgres=# \q
The first line is in your terminal, the next two are inside psql. Then do your rake db:create.

The User user is possibly a default but user is already taken for other purposes in PostgreSQL so 
you'd have to quote it to preserve the case if you wanted to use User as a username:

postgres=# create role "User" login createdb;