Connecting telegram bot to google spreadsheet

Question

function setWebhook() {
  var url = telegramUrl + "/setWebhook?url=" + webAppUrl;
  var response = UrlFetchApp.fetch(url);
  var obj = JSON.parse(response)
  Logger.log(obj);
}

function getUpdates() {
  var url = telegramUrl + "/getUpdates";
  var response = UrlFetchApp.fetch(url);
  Logger.log(response.getContentText());
}

function sendMessage(chat_id, text){
var url = telegramUrl + "/sendMessage?chat_id=" + chat_id + "&text=" + text;
var response = UrlFetchApp.fetch(url);
}


function doPost(e){
var contents = JSON.parse(e.postData.contents)
var chat_id = contents.message.from.id;

sendMessage(chat_id, "Hi, Message Recieved")
}

The problem is the doPost function. Google script keeps saying postData is a type error. Therefore, my sendMessage function does not work too. However, I don't know what is wrong with my codes.