API_DOQUERY Javascript is not working

More than likely this is the statement that needs to be corrected from:

var recordid = $("ItemID", xml).text();

to this:

var recordid = $("itemid", xml).text();

The XML tag is derived from the field label by lower-casing all letters and substituting an underscore for non-alphabetic characters. You should expand the xml variable in the console to see the full XML response:

console.dirxml(xml);

For what is is worth, I would make further improvements to your code such as the following:

(function() {
  var dbid = "myDBID";
  var apptoken = "oooolooktokens";
  var pro = kRid;
  var ntit = _fid_9").value;
  var nupc = _fid_75.value;
  $.ajaxSetup({data: {apptoken: apptoken}});
  var promise = $.get(dbid, {
    act: "API_DoQuery",
    query: '{35.CT.${ntit}}AND{34.CT.${nupc}}',
    clist: "3"
  }).then(function(xml) {
    console.dirxml(xml);
    var recordid = $("itemid", xml).text();
    console.log(recordid);
    _fid_74.value = recordid;
  });
})();

Also, you probably need to add decoding logic to determine what type of age you are on as I think you code is only going to work for add and edit pages.