First of all, make sure the database settings in your db-config.php are correct, specifically the name and password of your MySQL user, the name of your database, and the name of your MySQL server.
If you're running your own server, you may need to give your MySQL user proper permissions. Log in to MySQL as the MySQL root user and issue these commands:
GRANT ALL PRIVILEGES ON database_name TO user@host IDENTIFIED BY 'password';
FLUSH PRIVILEGES;
Replacing the lower-case strings with the actual values, of course.