Q: OAuth 2.0 Playground: unauthorized_client I've tried several times, but I can't use the Google PlayGround tool with the Google People API v1.
Request / Response
POST /oauth2/v4/token HTTP/1.1
Host: www.googleapis.com
Content-length: 278
content-type: application/x-www-form-urlencoded
user-agent: google-oauth-playground
code=4%2FhQlhA-MiWKhcmHWVUddb8TmiaVEDdMd_3lDHid9eYBc&redirect_uri=https%3A%2F%2Fdevelopers.google.com%2Foauthplayground&client_id=111243977462-pc15rhq33ojuc7i54ce3qd8upj6mtnc3.apps.googleusercontent.com&client_secret=ozWubBNz1iKdykitcK757UOo&scope=&grant_type=authorization_code
HTTP/1.1 401 Unauthorized
Content-length: 74
X-xss-protection: 1; mode=block
X-content-type-options: nosniff
Transfer-encoding: chunked
Expires: Sun, 16 Jul 2017 14:54:42 GMT
Vary: Origin, X-Origin
Server: GSE
-content-encoding: gzip
Cache-control: private, max-age=0
Date: Sun, 16 Jul 2017 14:54:42 GMT
X-frame-options: SAMEORIGIN
Alt-svc: quic=":443"; ma=2592000; v="39,38,37,36,35"
Content-type: application/json; charset=UTF-8
Www-authenticate: Bearer realm="https://accounts.google.com/"
{
  "error_description": "Unauthorized", 
  "error": "unauthorized_client"
}

I've already followed the tips below:
1. Delete the whitespace in the 'OAuth Client ID' and 'OAuth Client secret' in the OAuth 2.0 configuration of Google PlayGround
2. Define in the manager API the authorized redirection URI for: https://developers.google.com/oauthplayground
3. Verify in the Manager API is enabled on the Dashboard

Could someone help me with any more tips for me to try to solve the problem?
Below is a description of what I did on Google Playground:
Step 1 Select & authorize APIs
1. I select Google API v1 and framework https://www.googleapis.com/auth/contacts.readonly
2. Click the 'Authorize APIs'

Step 2 Exchange authorization code for tokens
1. Click the 'Oauth 2.0 Configuration'
2. Click on the 'Use your own OAuth credentials'
3. Enter the 'OAuth Client ID' and the 'OAuth Client secret'
4. Click the button: 'Exchange authorization code for tokens'

I do not know if it has to do with the issue, but I noticed that the list of applications connected to my account does not appear 'OAuth 2.0 Playground'. But I have the 'Google APIs Explorer' where I successfully tested access to my contacts (Google People API).

A: Go to settings by clicking the gear icon. Set the following as defined below and then tick the 'Use you own Oauth credentials' and fill in the client_id and client_secret of your Google OAuth app :

Add google.com in the authorized domain list of your app's OAuth consent screen.

Click on the application for which you want to configure for the next step:

Make sure to also add 'https://developers.google.com' in the Authorized JavaScript origins and 'https://developers.google.com/oauthplayground' in the Authorized redirect URIs[click 'save' below once added]:

Then click 'Authorize APIs' after selecting the appropriate access requirements:

You should get a prompt and then authorize it [incase of a safety warning, proceed ahead and click allow for the permissions requested] 
Once done you should have an authorization code using which you can generate your token[access token and refresh token, we get a refresh token as we specified 'offline' in the access-type settings earlier]. This is a one time auth code[you would get an invalid_grant if you try to re-use it], store the access token and refresh token for talking to google APIs, new access token can be generated using the refresh token.
