JSFiddle - React, Tailwind, and code Playground

JavaScript

var adsURL = new Array();
    adsURL[0] = "testpage0.htm";
    adsURL[11] = "testpage11.htm";
    
    var nextAd;
    
    var moveNextAd = function (top) {
        nextAd.style.top = top + ".px"
    }; //changed this to function as variable, answer explained below
    
    var changeBannerAd = function () {
        var num = adsURL.length
        var allAds = document.getElementById("bannerBox")
        allAds = allAds.childNodes;
        //^^^ here is what was wrong
        // Child nodes returns an array of elements
        // So for allAds, you should reference it with allAds[i]
        for (var i = 0; i < num; i++) {
            if (allAds[i].style.zIndex == 0) {
                allAds[i].style.top = "-50px";
                nextAd = allAds[i++];
            }
        }
    
        for (var i = 0; i < num; i++) {
            allAds[i].style.zIndex--;
    
            if (allAds[i].style.zIndex < 0) allAds[i].style.zIndex = num - 1;
        }
    
        var timeDelay = 0;
        for (var i = -50; i <= 0; i++) {
            setTimeout((function() {moveNextAd(" + i + ");}()), timeDelay);
            timeDelay += 15;
        }
    }; //changed this to function as variable too.
    
    function addEvent(object, evName, fnName, cap) {
        if (object.attachEvent) object.attachEvent("on" + evName, fnName);
        else if (object.addEventListener) object.addEventListener(evName, fnName, cap);
    }
    
    function makeBannerAds() {
        var bannerBox = document.createElement("div");
        bannerBox.id = "bannerBox";
        document.body.appendChild(bannerBox);
        for (var i = 0; i < adsURL.length; i++) {
            var bannerAd = document.createElement("div");
            bannerAd.className = "bannerAd";
            bannerAd.style.zIndex = i;
            bannerAd.style.position = "absolute";
            var urlLink = document.createElement("a");
            urlLink.href = adsURL[i];
            var bannerIndex =...