Below is the solution for your issue.
1.Go to the following path Facebook Profile - > Settings -> Apps and just Remove YOUR APP from it.
2.In your Developer Dashboard Go to your App Review -> Add Items
1.manage_pages (This Permission necessery for Accessing the page access tokens)
Add above the permission for review - with any of the simple video of above error
3.Now go to your Messanger-> Settings -> Token Generation Select page from the dropdown List now it will open the popup (because you have removed this app in the 1st step) for the app access permission , notice that now it will ask you for manage pages permission just Grant those permission and then create access tokens
4.Now try again to subscribe the Webhook to your Pages.