Trying to figure out why my child theme is trying to access a file that doesn’t exist

From your comment:

I found this code in the functions.php of the parent theme, is that what you were looking for?

// bootstrap 

wp_register_style('bootstrap', get_template_directory_uri() . '/css/lib/bootstrap.css'); > 
wp_enqueue_style('bootstrap');

1. In your Theme’s file structure, do you find /css/lib/bootstrap.css? If so, then it should be loading properly.
2. If not, do you find bootstrap.css anywhere else in your Theme’s file structure? If so, then you should change the wp_register_style() function accordingly, to reference the correct file location.
3. If not, do you intend to enqueue the bootstrap stylesheet? If not, then simply remove the code entirely.
4. Another thing: are you using a Child Theme? If so, then the Theme is looking for /css/lib/bootstrap.css in the Parent Theme, due to get_template_directory_uri(). If you have the files in your Child Theme, change that to get_stylesheet_directory_uri().

Edit

Also, the link doesn’t return a 404 for me. I can see the stylesheet, just fine.