FATAL:  role "postgres" does not exist

createuser -s -r postgres

Ex: createuser -s -U $USER
enter your required role name: postgres
enter password for your the user:

psql -U 'your_database_name'

$ sudo su
# su postgres
# createuser -s --username=postgres MYUSERNAME

sovanlandy=# CREATE ROLE postgres LOGIN;

sovanlandy=# ALTER ROLE postgres LOGIN;

$createdb myapp_development

default: &default
  adapter: postgresql
  encoding: unicode
  pool: 5
  host: <%= ENV.fetch("DATABASE_HOST") %>
  password: <%= ENV.fetch("DATABASE_PASSWORD") %>
  port: 5432

development:
  <<: *default
  database: exactpos_development

test:
  <<: *default
  database: exactpos_test

production:
  <<: *default
  database: exactpos_production
  host: <%= ENV.fetch("DATABASE_HOST") %>
  username: <%= ENV.fetch("DATABASE_USER") %>