You can create a Post under Website > Posts.
Then at the bottom make it Create a New Page.
This will allow you to create a full page article within there.

The "Post" is the listing of the article, which links to a "Page" where the full content of the article sits.

Here's our help documentation on this:
https://www.oncord.com/resources/learning/tutorials/website/creating-your-first-post/