This situation typically happens when a scheduled post is removed directly from the Facebook page.
Make sure to check if any of the administrators of the page on Facebook removed the post first. To do so, on Facebook, go to Publishing Tools, then Scheduled Posts (you must be an admin to be able to see this page).
If you don't see the post in this list, then it means someone deleted the post however, we still have to manually delete it from GAIN to be extra sure. Even if the post is there, please contact us so we can delete it from GAIN after you remove it from the Scheduled Posts list.
To avoid these types of issues in the future, make sure that all your posts scheduled through GAIN are deleted from GAIN. This way we can be 100% sure the post will not be published.