postgresのパスワードを設定しましたか ユーザー？
config/database.ymlで データベースを適切に設定する必要があります：
development:
adapter: postgresql
encoding: unicode
database: your_app_development # name your development app something
host: localhost
pool: 5
username: your_username
password: your_password # or leave blank if you didn't set a password
test:
adapter: postgresql
encoding: unicode
database: your_app_test
host: localhost
pool: 5
username: your_username
password: your_password