This error:
"This method must be called with an app access_token."
is quite straight-forward. You just need to make this API call with an App access token, instead of a normal access token (user token). To use it with your call simply add another parameter: access_token
and make the call.
The App Access token is nothing but, APP_ID|APP_SECRET
, or you can make the call (ref.)-
GET /oauth/access_token?
client_id={app-id}
&client_secret={app-secret}
&grant_type=client_credentials
But beware, its not recommended to expose app access token
on the client side, since its like a password of your app.