I am trying to post on the wall of a facebook page. I am able to post on the user wall using App Access token.
I got the App Access Token through extending the Defau
The App Access Token is the most basic one, and will not allow you to post anything. In order to post something to a Facebook Page (as a Page), you need to get a Page Access Token.
The process is a bit complicated, because you actually need to authorize the user with the "manage_pages" permission first, with the User Access Token you can call the API to get a Page Access Token (/me/accounts).
See those links:
Btw, the REST API is deprecated: https://developers.facebook.com/blog/post/616/
You can also try the "Client Token" (Developer Settings > Advanced), i never worked with that one but it could be the easiest solution. In any case, an App Access Token is the wrong one.