Wordpress fatal error help needed

Hi!

I am a beginner with Wordpress.
I am now stuck with an error. See file attached, or this:

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in /var/www/html/wordpress/wp-includes/wp-db.php:1688 Stack trace: #0 /var/www/html/wordpress/wp-includes/wp-db.php(632): wpdb->db_connect() #1 /var/www/html/wordpress/wp-includes/load.php(558): wpdb->__construct('root', 'password', 'test_db', 'db') #2 /var/www/html/wordpress/wp-settings.php(124): require_wp_db() #3 /var/www/html/wordpress/wp-config.php(96): require_once('/var/www/html/w...') #4 /var/www/html/wordpress/wp-load.php(50): require_once('/var/www/html/w...') #5 /var/www/html/wordpress/wp-admin/admin.php(34): require_once('/var/www/html/w...') #6 /var/www/html/wordpress/wp-admin/themes.php(10): require_once('/var/www/html/w...') #7 {main} thrown in /var/www/html/wordpress/wp-includes/wp-db.php on line 1688

This happened after I created a new folder in themes.
I added 2 files: front-page.php, and blog_posts.php
I then added code from a pdf-file (I do not have that code here, but I can get it again if needed)

I use docker to handle php-admin, apache server, Wordpress, etc.

Need help with this asap! Thanks.