This Facebook Page is no longer available for your account to subscribe to
This happens when a particular Facebook page that you were previously subscribed to has either been deleted or your access to a page has been disabled. When this happens you'll see the error below.
If you need access to this page, you'll need to contact the Facebook Page owner to grant you permission to the page you were previously subscribed to. Once permission has been granted, the Facebook page can be connected again. Here are step-by-step instructions: Facebook Lead Add Direct Integration.
If this page is no longer needed for lead processing, click Update Selected Pages to reset the connection and the expired Facebook page will be removed from your Facebook integration page.