The user must be an administrator of the page in order to impersonate it
This error most likely occurs when the Facebook administrator who connected the page to Sprout Social has been removed as an admin by the Facebook page owner. When this occurs, Sprout no longer have access to post in the platform. A current Facebook admin must reconnect and reauthorize the page in Sprout. If you are unsure of who connected the page originally, please contact firstname.lastname@example.org.
Note: In order to reconnect a Facebook page, you must be the Sprout Account Owner or have the Manage profiles & permissions privilege, as well as be an admin for the Facebook Page you want to connect.