If you receive a notification to tell you that your post has been unapproved, there could be a few different reasons. But in general it will be because the post contains content that is deemed inappropriate.
A. For the overall brand i.e. it is off brand, colours, images etc
B. For the campaign it is associated with
C. It contains content that potentially the company does not want to make public
D. Bad spelling or grammar
As of Jan 2019 you cannot edit the post and re-send for moderation, however this functionality will be available by Q2, 2019.