When creating a Facebook ad, you will have the option to select an organic Page post to promote.
If you are not seeing your most recent organic Page posts, it may not have imported yet.
Any changes that were made in the Facebook interface will be imported to your account approximately every 2-6 hours.
If you published a recent post on your Facebook Page, it will show up after the import completes. You can always check your last import in the Integrations tab.