This is a known issue with Facebook.
If you created a new Facebook page or a new Facebook group it may not appear under your RecurPost account.
The reason for this is that when you gave access to RecurPost, you did not select the newer page or group as this did not exist back then. Hence, it will work as if you did not give permission to this page or group.
How to fix this issue?
It is rather easy to fix this though.
You can go to your Business Integration under Facebook settings.
If you are logged in to Facebook, simply go to https://www.facebook.com/settings?tab=business_tools
On that page, click on “view and edit” link under RecurPost app.
Scroll down on the pop up and check the box next to the page or group that you added recently. As soon as you do that, go back to RecurPost and try to add it now. It should show up in the list.