Problem migrating from single to multisite

I am having problems getting multisite working.

My goal is to be able to manage two websites with one instance of WP on a laptop. I publish static versions to services that serve static pages.

I have WP 5.2.3 installed, and have been working to convert from a single-site to a multisite.

I now have Tools>Network Setup available.

I get a page stating “Warning: An existing WordPress network was detected.”

The page https://localhost/wordpress/wp-admin/network.php

Now instructs me to add, in wp-config.php, the following lines just above the existing line

/* That’s all, stop editing! Happy publishing. */

— text to add

define(‘MULTISITE’, true);
define(‘SUBDOMAIN_INSTALL’, false);
define(‘DOMAIN_CURRENT_SITE’, ‘localhost’);
define(‘PATH_CURRENT_SITE’, ‘/wordpress/’);
define(‘SITE_ID_CURRENT_SITE’, 1);
define(‘BLOG_ID_CURRENT_SITE’, 1);

(The line “define(‘MULTISITE’, true);” was already there, when WP worked.)

After adding these lines and saving the file and attempting to log in again, I get a page stating

Error establishing a database connection

What should I do next?