The error says that osTicket cannot talk to the Database. You can check your PHP error logs to see if there is more information there. You can also check your MySQL server error logs.
Check the /include/ost-config.php to make sure that the username/password and database name are correct.
Then go to your database server and ensure that the user exists and the password is correct. Then ensure that the MySQL user has access to the database.
Lastly if your webserver and database server are different machines make sure that the user has access from the web server that osTicket resides on. If they are separate machines then you might want to also check your security software to ensure that the ports / service are not blocked.