slach command failed with "dispatch_failed"

This is the very simple code using bot of GAS. e.g., /test hello -> bot: hello

But the error appears, why?

function doPost(e) {
  array = e.parameter.text.split(" ");
  if (array.length > 1) {
      postSlack(array[1], e.parameter.user_id);
  }
}

function postSlack(text, userId) {
  var url = "incoming url";  
  var payload = {
    text: "<@"+ userId + "> " + text
  };
  var options = {
    "method" : "POST",
    "headers": {"Content-type": "application/json"},
    "payload": JSON.stringify(payload)
  };
  UrlFetchApp.fetch(url, options); 
}

/test failed with the error "dispatch_failed"