Problem:

Either the MySQL database information you used for installation are incorrect, or the MySQL service is not running on the server.

Solution:

Please perform the following checkings -

The MySQL service is started and running on your online server. You may wish to check it and make confirmation with your server administrator.
Verify that the entire MySQL host, MySQL username, MySQL password and MySQL database info are correct.
Access to your MySQL database, please make sure that the MySQL user is granted Full Privileges upon the MySQL database.