Validating webhook endpoint in Google Apps Script

API and Webhooks
1

I’d like to use Google Apps Script to process events. But I can’t get past the validation of the endpoint, I keep getting “URL validation failed. Try again later.” Here’s the code.

function doPost(e) {
    // Parse the JSON payload from the request
    var requestData = JSON.parse(e.postData.contents);
    if (requestData.event === "endpoint.url_validation") {

      // Extract the plainToken from the payload
      var plainToken = requestData.payload.plainToken;

      // Create HMAC SHA-256 hash using the secret token as the key and plainToken as the message
      var encryptedToken = Utilities.computeHmacSha256Signature(plainToken, WEBHOOK_SECRET_TOKEN);

      // Convert the byte array to a hex string
      encryptedToken = encryptedToken.map(function (byte) {
        var hexString = (byte & 0xFF).toString(16);
        return (hexString.length === 1) ? '0' + hexString : hexString;
      }).join('');

      // Create a response object with both tokens
      var response = {
        plainToken: plainToken,
        encryptedToken: encryptedToken
      };

      // Convert the response object to JSON
      var jsonResponse = JSON.stringify(response);

      // Return the JSON response
      return ContentService.createTextOutput(jsonResponse).setMimeType(ContentService.MimeType.JSON);
   }
}

It executes without errors.
What am I doing wrong?