Check if a selector is on the page, then set a cookie

I would like to set a cookie if a certain selector is present on the page.

      function createCookie(name,value,days) {
    if (days) {
        var date = new Date();
        date.setTime(date.getTime()+(days*24*60*60*1000));
        var expires = "; expires="+date.toGMTString();
    }
    else var expires = "";
    document.cookie = name+"="+value+expires+"; path=/";
} createCookie("internal", "true",1000);

How do I use if else in this case?

I tried something like this:

function createCookie(name,value,days) {
if (document.querySelector("#top > div.LandbotLivechat-container > div > div.LandbotProactive")){
if (days) {
        var date = new Date();
        date.setTime(date.getTime()+(days*24*60*60*1000));
        var expires = "; expires="+date.toGMTString();
    }
    else var expires = "";
    document.cookie = name+"="+value+expires+"; path=/";
}
  createCookie("chat seen", "true",1000);
  }

I am working in a HTML tag within Google Tag Manager