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

$(document).ready(function() {
    $("#chkdwn2").click(function() {
       if ($(this).is(":checked")) { 
          $("#dropdown").prop("disabled", true);
       } else {
          $("#dropdown").prop("disabled", false);  
       }
    });
});