You need to confirgure the database in CPanel so that phpbb will see the database and write to it. In CPanel it is shown as the menu "Manage Mysql" ... demo here http://www.demodemo.com/1/tutorials/cpanel_bluelagoonx_mysql.htm
Make sure that you get the correct full name of the database, which will be something like "phpbb_mydatabase".
Make sure also that you have the correct path to the database.