Posting on my facebbok page from my facebook app - Facebook login doesn't want to grant permission

I have a facebook page (I'm the admin) and a facebook app (I'm the developer) and through this app I want to post on this page.

Via http://ift.tt/OJg2cb I'm trying to grant to my app the manage_pages permission to my page. But an error pops up: "Some of the permissions below have not been approved for use by Facebook. Submit for review now or learn more."

If I click on "ok" I obtain the access token (and then the fb_exchange_token) but, as the pop up says, something goes wrong: the app isn't able to post on the page ("The user hasn't authorized the application to perform this action").

Why?

Thank you!