Why is this LockService not working as expected when sending email?

Question

I'm using LockService when sending emails, but when multiple users run it at the same time, it throws the error: Service invoked too many times...

Here's some log, showing the failures:

function sendEmail() {
  const ss = SpreadsheetApp.getActiveSpreadsheet();
  var lock = LockService.getScriptLock();
  try {
    lock.waitLock(3000); // wait 03 seconds for others' use of the code section and lock to stop and then proceed
  } catch (e) {
    Logger.log('Someone has just sent an Email. Try it again 3 seconds later.');
    return HtmlService.createHtmlOutput("<b> Server Busy please try after some time <p>")
    // In case this a server side code called asynchronously you return a error code and display the appropriate message on the client side
    //return "Error: Server busy try again later... Sorry :("
  }
  
   /*
   Gets certain data from current row. These are used as criteria
   */

    GmailApp.sendEmail(email, "Text", name + " BODY.", { name: 'Diplay Custom Name as Sender' });
    
//Looks for a matching record in another sheet to mark its adjacent column as sent ("Yes")
    for (var n = 0; n < formRespValues.length; n++) {
      if (formRespValues[n][1] == testNo) {
        formRespSheet.getRange('M' + (2 + n)).setValue('Yes');
      }
    }
  }
  SpreadsheetApp.flush(); // applies all pending spreadsheet changes
  lock.releaseLock();
}