Can’t extend some core classes

I am relatively new to wordpress development, I am trying to extend ‘Walker_Category_Checklist’ class found in /wp-admin/includes/class-walker-category-checklist.php to customize it for wp_terms_checklist() or wp_category_checklist() but when I try to require_once a link to my extended class in the child-theme function.php I get this error and site not loading!

filemtime(): stat failed warning within a child theme

I am trying to work on my child theme, but I can’t get rid of the filemtime(): stat failed warning, which appeared as soon as I activated my child theme.
After reading other questions, I understand that the filemtime warning is often caused by incorrect use of URLs and paths. However, I think I provided URLs and paths correctly, and if that’s the case, then there is something else I’m missing.