Sorry, but your question is not clear. This may be a problem specific to your theme. If so, you should post the question on the support forum for the theme.
Be sure to explain a bit more and post a link to your site showing the page you want to change?
You cannot add a post to a page. Posts and pages are separate. That much is clear.
You can add a child page to a page. To do that, create a new page and in the “Page Attributes” box specify its parent page. The child page might appear in the main menu automatically, or you might have to add it manually, depending on your theme and how you have customized it.
