NodeJS Facebook User Feed Subscriptions: Invalid OAuth access token

Am close to tearing my hair out (as a passionate developer, should), so hopefully I can get some help on this one. After reading a bunch of forums and sifting through facebook documentation, here is a log file of trying to make a subscription to user feed's. Essentially, making a DELETE subscription request , requesting for an application access token and finally trying to make the subscription

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makeRequest - Host: graph.facebook.com - Path: /v2.2/{ app_id }/subscriptions?object=user
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Type: DELETE
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Wed Feb 25 2015 12:52:15 GMT+0000 (UTC) Server is listening on port 81
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makeRequest - Host: graph.facebook.com - Path: /oauth/access_token?client_id={ app_id }&client_secret={ app_secret }&grant_type=client_credentials
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Type: GET
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App Access Token: { app_token }
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makeRequest - Host: graph.facebook.com - Path: /v2.2/{ app_id }/subscriptions?&object=user&fields=feed&callback_url={ uri_encoded_ callback_url }&verify_token={ verify_token }&access_token={ app_token }
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Type: POST
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Success: {"error":{"message":"Invalid OAuth access token.","type":"OAuthException","code":190}}

Hopefully someone spots what obvious mistake I'm making!

Thanks in advance!