As I understand you're using an App AccessToken for an endpoint (object type) which needs a user AccessToken. See the docs at https://developers.facebook.com/docs/graph-api/using-graph-api/#search
It's stating
"All Graph API search queries require an access token included in the request. The type of access token you need depends on the type of search you're executing. Searches across Page and Place objects requires an app access token. All other endpoints require a user access token."