SOLVED: issue with Generate you Access Code. This is what the page says from Google: (FYI - no extraneous spaces for Client ID or Secret)
400. That’s an error.
Application: Elite Agents Network
You can email the developer of this application at: firstname.lastname@example.org
The redirect URI in the request, urn:ietf:wg:oauth:2.0:oob, can only be used by a Client ID for native application. It is not allowed for the 'WEB' client type. You can create a Client ID for native application at https://console.developers.google.com/apis/credentials/oauthclient
That’s all we know.
NOTE: Kaisa already helped me brilliantly. I had incorrectly created credentials for a web browser when the correct selection is Other - UI. Everything is working perfectly now.
Refer to here answer here: https://premium.wpmudev.org/forums/topic/appointments-google-calendar-api-generate-your-access-code-link-broken#post-1100874