There are a few possible reasons you received this error message:
1. The user who made the post has removed the message from Facebook. You are attempting to comment on a post that no longer exists.
2. A page moderator has removed the post from Facebook. You are attempting to comment on a post that no longer exists.
3. The user has updated their Facebook Privacy settings in a way that prevents third party systems from interacting with their data. This change would have occurred after Sprout Social collected the original post. Now as you are responding to the post from Sprout Social, the privacy setting is blocking you from doing so. For more information on these privacy settings, click here.