banner in post

I don’t see anything wrong with the code you added. If that’s your child theme functions.php file then you need to remove the “load” line at the top. This looks like a copy of the parent theme functions.php file and, if this is in your child theme, you don’t want to load the init-core.php file twice. If this is the parent theme functions.php file then I would recommend you remove your filter/function from that file and place it in your child theme functions.php file; otherwise you’ll lose your changes when you update the theme.

Also, this shouldn’t make any difference but you might try enclosing the href and src arguments in quotes:

$('.single .post .entry').append($('<div align=center><a href="https://bgtrekking.com"><img src="https://planinar.org/store/banners/trekking.jpg alt=trekking/"></a></div>'));