JSFiddle - React, Tailwind, and code Playground

HTML

<select id="dropdown" style="width:200px;">
              <option value="feedback" name="aft_qst">After Quest</option>
              <option value="feedback" name="aft_exm">After Exam</option>
              </select>

              <input type="checkbox" id="chkdwn2" value="feedback"/>

JavaScript

$("#chkdwn2").change(function() { 
    if (this.checked) $("#dropdown").prop("disabled",true);
    else $("#dropdown").prop("disabled",false);
})