Defina a senha para o usuário do Postgres Ubuntu
sudo -u postgres psql
ALTER USER <myuser> WITH PASSWORD 'mypassword';
Classy Answer
sudo -u postgres psql
ALTER USER <myuser> WITH PASSWORD 'mypassword';ALTER USER user_name WITH PASSWORD 'new_password';sudo -u postgres psql postgres
postgres=# ALTER USER postgres PASSWORD 'myPassword';
ALTER ROLE
\password postgres