这将工作：

var reqs_id = 0;

function removeElement(ev) {
  var button = ev.target;
  var field = button.previousSibling;
  var div = button.parentElement;
  div.removeChild(button);
  div.removeChild(field);
}

function add() {
  reqs_id++; // increment reqs_id to get a unique ID for the new element

  //create textbox
  var input = document.createElement('input');
  input.type = "text";
  input.setAttribute("class", "w3-input w3-border");
  input.setAttribute('id', 'reqs' + reqs_id);
  input.setAttribute('value', reqs_id);
  var reqs = document.getElementById("reqs");
  //create remove button
  var remove = document.createElement('button');
  remove.setAttribute('id', 'reqsr' + reqs_id);
  remove.onclick = function(e) {
    removeElement(e)
  };
  remove.setAttribute("type", "button");
  remove.innerHTML = "Remove" + reqs_id;
  //append elements
  reqs.appendChild(input);
  reqs.appendChild(remove);
}

<div id="reqs">
  <h3 align="center"> Requirements </h3>
</div>
<button type="button" value="Add" onclick="javascript:add();"> Add</button>

从我以前的答案修正。另一个可能是必要的选项是让每个元素知道它的确切位置，并能够根据添加或删除的内容调整自己。此增强将通过重新调整并确保您的元素始终处于有序状态来解决这个问题。（如果需要）
参见JSFiddle示例。
HTML

<div id="reqs">
   <h3>Requirements</h3>
   <button type="button" value="Add" onclick="javascript:add();">Add</button>
   <br>
</div>

简体中文

function removeElement(e) {
    let button = e.target;
    let field = button.previousSibling;
    let div = button.parentElement;
    let br = button.nextSibling;
    div.removeChild(button);
    div.removeChild(field);
    div.removeChild(br);

    let allElements = document.getElementById("reqs");
    let inputs = allElements.getElementsByTagName("input");
    for(i=0;i<inputs.length;i++){
        inputs[i].setAttribute('id', 'reqs' + (i+1));
        inputs[i].setAttribute('value', (i+1));
        inputs[i].nextSibling.setAttribute('id', 'reqsr' + (i+1));
    }
}

function add() {
    let allElements = document.getElementById("reqs");
    let reqs_id = allElements.getElementsByTagName("input").length;
    reqs_id++;

    //create textbox
    let input = document.createElement('input');
    input.type = "text";
    input.setAttribute("class", "w3-input w3-border");
    input.setAttribute('id', 'reqs' + reqs_id);
    input.setAttribute('value', reqs_id);
    let reqs = document.getElementById("reqs");

    //create remove button
    let remove = document.createElement('button');
    remove.setAttribute('id', 'reqsr' + reqs_id);
    remove.onclick = function(e) {
        removeElement(e);
    };
    remove.setAttribute("type", "button");
    remove.innerHTML = "Remove";

    //append elements
    reqs.appendChild(input);
    reqs.appendChild(remove);
    let br = document.createElement("br");
    reqs.appendChild(br);
}