scrolltop always 0, can’t trigger script

I guess

body.scrollTop is deprecated in strict mode. Please use documentElement.scrollTop if in strict mode and body.scrollTop only if in quirks mode.

So I would suggest you to use document.documentElement.scrollTop. also let us know when are you using this statement? show us full code and your console error if possible

Please try the below code edited with respect to your comment code and let us know what you see in console.

jQuery(document).ready(function ($) {
 jQuery(window).scroll(function(){
 var ScrollTop = jQuery(window).scrollTop();
 if (ScrollTop > 100) { 
  jQuery( ".site-header" ).addClass( "scroll" ); 
   console.log("above 100");
 }else{
 jQuery( ".site-header" ).removeClass( "scroll" ); 
   console.log("below or equal to 100");
 }
 });
});