A post page is very similar to a tag page, but it's a dedicated page where you'd create a collection manually.
Create your post page like a Pro.
After you create a page, you will see a page with a 'No post available' sign. That's right – as you haven't assigned any posts yet, it's showing no post.
Assign your posts, and create a gallery.
Go to your dashboard, and click 'Edit' on your post. You'd see a 'Choose your page' dropdown menu. Assign a page, and click 'Edit'.
Now it's all there!
Assigning your post does not remove the original post. It's still visible in your 'Post all' page.