Error establishing a database connection

Database connection credential are wrong, check if database host, user and password are correct.

Try to add a new user to the database and update wp-config.php.

This is usually a problem with MySQL on your host’s end of things, though can be a result of changing your db username or password.

Take a look at this tutorial for a good rundown of possible solutions:

https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-error-establishing-a-database-connection-in-wordpress/