What is an existing page post?
In the campaigns section, you can select an existing Facebook post to put a campaign behind!
When creating a campaign, you have the ability to select an existing page post to put the campaign behind. This step is completed toward the end of the campaign creation segment where it will say "Properti Creative" and "Existing Page Post".
If the existing page post is not displaying in this segment, that is because it has been deemed "un-promotable" meaning that Facebook will likely reject the post for the campaign.