Flutter ''http'' always giving me 503 status code

I create an api in node.js and It's working fine in postman but when I'm trying to post data in flutter it's giving me 503 status code.

https://buuzuu.herokuapp.com/save

{
"email" : "abc@ab.com",
"name" : "James",
"message" : "Cio Cio"
}

This is my flutter code. I am calling this method on a button click.

postData() async {
var url = 'https://buuzuu.herokuapp.com/save';
var response = await http.post(url, body:
{
  "email" : "abc@ab.com",
  "name" : "James",
  "message" : "Cio Cio"
}
);
print(response.statusCode);
}