You can't assign pages you have created to the Category Special Page. All the special pages are already predetermined by Wordpress. For example when you set up a page via the Static Page option in Settings > Reading for your Post Page (blog) this page become the Blog special page.
When you create a category, by going to Posts > Category, these are considered special pages as well.
The below information is taken from our Special Page documentation:
Special pages refer to the WordPress pages which dynamically display data from your site. These Special Pages are Blog Page, Archive Page, Category Page, Search Results, Tag Listing, Author Posts and 404 Page.
For example, your Blog Page is a Special page, because it takes all of your individually entered posts and displays them on one page. Another example of a Special page is an Author Posts page which displays all the posts by an author. Category pages display all the posts in a category.
If you wish to personalise the category pages, you can do so via Drag & Drop > Category template, then configure the sections from the Page Options > Category. However, if you want to add specific content on a per category page for example,
Lets say you have a category called Apples and Oranges and on on the Apples category page, you want to add a banner image that has apples and then do the same for the Orange category page but instead of an banner image with apples you want one with oranges. You will need to use a hook(s).
For more information hooks, see the Wordpress Codex article -
Also WP Candy has a great post about them too -
There is also a plugin called ActionMap which is by PageLines and will assist you in find the correct location for your hook(s).
You will also need to use the following to target your categories.
If this seems a little too difficult, then I recommend contacting one of our Pros, who will be happy to assist you.