psql: error: FATAL: role "postgres export" does not exist code example

Example 1: role "postgres" does not exist

template1=# CREATE USER username WITH PASSWORD 'user_password';
template1=# GRANT ALL PRIVILEGES ON DATABASE "user_database" to username;

Example 2: psql: error: FATAL: role "postgres" does not exist

createuser -U postgres -s YOURUSERNAME
createdb YOURUSERNAME