Error 400: invalid_scope Some requested scopes cannot be shown: [https://www.googleapis.com/auth/homegraph] - google-api

Question

So I'm trying to use the Home Graph API by calling the API endpoint
https://homegraph.googleapis.com/v1/devices:requestSync
It is a HTTP POST request and it needs an ACCESS_TOKEN and service account key.
Getting the service account key is easily done as per Google's documentation. The issue is getting the ACCESS_TOKEN.
As per this documentation by Google, I need to get ACCESS_TOKEN created using the following scope of permissions
https://www.googleapis.com/auth/homegraph
I opened OAuth 2.0 Playground to request a developer temporary ACCESS_TOKEN for testing. I wrote all the necessary urls and in scope I wrote this-
scope is written to be authorized
Now after this, I am navigated to my Authorization URL (ie, Google's sign in page). I login with email id and password.
If credentials are correct and scope mentioned is valid then I should have been redirected to OAuth playground page with authorization code which I would have exchanged for access token and refresh token.
But, what actually happens is after I enter my credentials, I get following error and I am never redirected to Oauth Playground page-
Authorization Error
Error 400: invalid_scope
Some requested scopes cannot be shown: [https://www.googleapis.com/auth/homegraph]
Request Details
access_type=offline
o2v=2
response_type=code
redirect_uri=https://developers.google.com/oauthplayground
prompt=consent
client_id=xxxxxxxxx.apps.googleusercontent.com
scope=https://www.googleapis.com/auth/homegraph**
I searched a lot online too, but couldn't find the actual reason.
So due to this issue with scope, I am not able to get ACCESS_TOKEN.
I have followed Google's documentation and the scope was mentioned there.
This is the pic from oauth 2.0 playground settings- OAuth 2.0 configuration

Answer 1

The issue is that you, a user, should not be getting and sending an access token. The service account should be getting and sending an access token. This is to make sure your service is authorized to talk to the Home Graph API.
You indicated you logged into the OAuth playground with "userid and password". But service accounts don't have passwords.
If you are using one of Google's libraries, it will take care of getting the access token for you, and this is the easiest way to do so. If you are just testing and need an access token, you can use something like oauth2l to get the access token based on the service account credentials.

Answer 2

I had implemented the REST approach to call HomeGraph Report State as below.
We need to follow the below steps:
Create a service account for your project and safely store the json file
Using the service account JSON, get the access token from Google
Using Oauth 2.0 token as Bearer authorization, invoke Report State API
Step 1:
This is straightforward. Please follow the steps in the below link
https://developers.google.com/assistant/smarthome/develop/report-state#expandable-1
Step 2:
Refer below code to get the Access token using service account json
GoogleCredentials credentials = GoogleCredentials
.fromStream(Helper.class.getClassLoader().getResourceAsStream("smart-home-key.json"))
.createScoped("https://www.googleapis.com/auth/homegraph");
credentials.refreshIfExpired();
AccessToken token = credentials.getAccessToken();
return token.getTokenValue();
Step 3:
Invoke Report State API
curl -X POST -H "Authorization: Bearer [[Access token from Step 2]]"
-H "Content-Type: application/json"
-d #request-body.json
"https://homegraph.googleapis.com/v1/devices:reportStateAndNotification"
Reference Links :
https://developers.google.com/assistant/smarthome/develop/report-state#http-post
https://cloud.google.com/endpoints/docs/openapi/service-account-authentication
https://developers.google.com/identity/protocols/oauth2/service-account#httprest_1
https://developers.google.com/assistant/smarthome/develop/report-state#expandable-1