You are sure you entered right database name, database user and server but still show error "Error establishing a database connection" on the front, what the reason ? If you met that in a multisite WordPress installation then it may caused by wrong domain name setting in you database.
- Go to the dabhoboard such as http://localhost/wordpress-401-mu/wp-admin you will see the following screen.
As the screen printed it cannot find out the site localhost/wordpress-401-mu/ in your database table mur_blogs ( if you use default database prefix then it is wp_blogs ).
Follow the instruction and open you database with phpmyadmin( or other database management tool ), and open the table mur_blogs ( default wp_blogs ). Plase change the domain & path to correct as the follow.
- Because you use localhost as the domain in you XAMPP, so domain always should be localhost, and the primary site may be localhost/your-directory-name/sub_site/
Posted from my blog with SteemPress : https://waytowp.com/wordpress-multisite-error-establishing-database-connection/