Proximity Beacon API, HTTP/1.1 403 Forbidden error

I've been reading about the Proximity Beacon API and running through the tutorial. This part of the tutorial makes use the OAuth 2.0 Playground. I believe I've followed the tutorial as described but when I get to step 3 and press send, I got HTTP/1.1 403 Forbidden error

HTTP/1.1 403 Forbidden
Content-length: 104
X-xss-protection: 1; mode=block
X-content-type-options: nosniff
X-goog-trace-id: 289418dcecc335c0fefb7456f402b0b5
Transfer-encoding: chunked
Vary: Origin, X-Origin, Referer
Server: ESF
-content-encoding: gzip
Cache-control: private
Date: Fri, 02 Jun 2017 07:28:32 GMT
X-frame-options: SAMEORIGIN
Alt-svc: quic=":443"; ma=2592000; v="38,37,36,35"
Content-type: application/json; charset=UTF-8
{
  "error": {
    "status": "PERMISSION_DENIED", 
    "message": "Unauthorized.", 
    "code": 403
  }
}

In my google developers console I have the Google Proximity Beacon API enabled. When Iam sending the request , iam getting the above type of error.

I must have missed something, but I'm not seeing it. Could anyone advise?

Regards,

Rajashekar