google.script.run.WithSuccessHandler returning the script completed but did not return anything

I am not seeing where the issue is in my code, that prevents it from returning the success message. I want to give me the message that the submission has been successful and to refresh the screen. While the data is being successfully submitted to the google sheet, I am getting the generic message that the script is completed but didn’t return anything.

var ss = SpreadsheetApp.openById('XXXXX');
var sheet = ss.getSheetByName('Messages');
var thisDocumentURL =ss.getUrl();
var ShtAdmin = "[email protected]";


function doPost(e){
  // Replace with your spreadsheet's ID.
  
  // name, phone_number and e-mail are form elements.
  var area = e.parameter.area;
  var assembly = e.parameter.assembly;
  var pass = e.parameter.pass;
  var line = e.parameter.line;
  var operator = e.parameter.operator;
  var shift = e.parameter.shift;
  var issueDescription = e.parameter.issueDescription;
  var personNotified = e.parameter.personNotified;
  var actionTaken = e.parameter.actionTaken;
  var personTakingAction = e.parameter.personTakingAction;
  var problemCorrected = e.parameter.problemCorrected;
  var approvalToContinue = e.parameter.approvalToContinue;
  var issueCategory = e.parameter.issueCategory;
  var id = getRandom();
  var timestamp = new Date(); // current date as epoch number
  

  sheet.appendRow(
    [id,timestamp,area,assembly,pass,line,operator,shift,issueDescription,personNotified,actionTaken,personTakingAction,problemCorrected,approvalToContinue,issueCategory,message]
  );
  
  var subject = "PEP Log Update";
  var header = "<p> This is an Automated mailer.  All Responses are directed to " + ShtAdmin+",</p>";
  var message = "<p><b>There has been an addition to the PEP log on "+ timestamp + ". </b></p>"+
                "<p><b>Associated with "+assembly +" running on Line " + line + ", "+pass+" pass. </p></b>";

MailApp.sendEmail({

  to: ShtAdmin,
  noReply: true,
  subject: subject,
  htmlBody: header + message 
})

onSuccess(message);
  
var pubUrl = ScriptApp.getService().getUrl();


}  //end of doPost Function


function getSuccessMessage(message){
  //Logger.log(data);
  return("PEP Successfully submitted.");
}

function onSuccess(){
  getSuccessMessage();
  var message = "You have successfully submitted the PEP.  Please refresh screen to reset form.";
  return message;
}



HTML Portion

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=chrome">
    <link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/ css/add-ons1.css" />
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"  crossorigin="anonymous" ></script>
    <title>Google Apps Script Tester</title>
  
</head>

<body>
    <form method="post" action= "<?= pubUrl ?>" >
    <form id="myForm">
             
        <label for="area">Select Area:</label>
        <select name="area" id="area">
        <option value="AOI">AOI</option>
        <option value="SMT">SMT</option>
        </select>
        <br><br>    
        Assembly: <input type="assembly" name="assembly" value=""><br><br>
        
        <label for="pass">Pass:</label>
        <select name="pass" id="pass">
        <option value="1st">1st</option>
        <option value="2nd">2nd</option>
        <option value="Single">Single</option>
        </select><br><br>
        <label for="line">Line:</label>
        <select name="line" id="line">
        <option value="Line 1/2">Line 1/2</option>
        <option value="Line 3">Line 3</option>
        <option value="Line 4">Line 4</option>
        <option value="Line 7">Line 7</option>
        <option value="Line 8">Line 8</option>
        <option value="Line 10">Line 10</option>
        </select>
        <br><br> 
        Employee Number: <input type="operator" name="operator" value=""><br><br>
        <label for="shift">Shift:</label>
       
        <label for="problemCorrected">Issue Corrected:</label>
        <select name="problemCorrected" id="problemCorrected">
        <option value="Yes">Yes Continue to Run</option>
        <option value="No-Escalate to Supervisor">No - Escalate to Supervisor</option>
        <option value="No-Conintue to Run with Approval">No - Continue to run with Approval</option>
        </select><br><br> 
        Approval to run w/o resolving issue: <input type="approvalToContinue" name="approvalToContinue" value=""><br><br>
        <label for="issueCategory">Issue Category:</label>
        <select name="issueCategory" id="issueCategory">
        <option value="Material Problem">Material Problem</option>
        <option value="Equipment Issue">Equipment Issue</option>
        <option value="Human Error">Human Error</option>
        <option value="Other">Other</option>    
        </select><br><br> 
        
        <br><br><br>
        <p>Please Click to submit data.</p>              
        <button type="submit" id="mySubmit" onClick="">Send Data</button>
        <p id="response_message"></p>
        
<script>
      const scriptURL = <?= pubUrl ?>
      const form = document.forms['myForm']
      google.script.run.withSuccessHandler(onSuccess);
      function onSuccess(message){
        var div = document.getElementById('response_message');
           }
      return response_message;
     form.addEventListener('submit', e => {
      e.preventDefault()
      var response_message = document.getElementById("response_message");
      fetch(scriptURL, { method: 'POST', body: new FormData(form)})
      .then(response => response_message.innerHTML = "Success!  Refresh screen for new form")
      .catch(error => response_message.innerHTML = "Error!")

    
        }
      })
    </script>

  
        



</form>
</form>

</body></html>