Buffer needs all permissions to be accepted when connecting a Facebook page or your posts will fail to publish. These are the most common errors you may see on failed posts that indicate that the Publish as Pages you manage permission wasn't accepted during the connection process:
An unknown error has occurred, please get in touch if this persists.
or
Yoiks! Buffer doesn’t have permission to post on your behalf. Please try reauthorizing this account by clicking reconnect under settings.
This can be resolved by refreshing your Facebook account, making sure you're signed into the correct Facebook profile when refreshing. If that doesn't do the trick, we recommend completely reauthorizing your Facebook page, following the steps here, to ensure all required permissions are accepted.
After reauthorizing, you should be all set and you can retry any failed posts in your queue.