Post categories to pages

Generally, pages cannot be added to categories or tags by default. Posts can can be categorized and tags.

If you google search “WordPress categorize pages” at least a couple of plugins can probably help you figure it out.

Otherwise, you can check out the code for some of those plugins to see how it’s done and do it yourself, fairly simply. The main thing to note is that you’re adding pages to the list of post types that categories and tags can be associated with:

register_taxonomy_for_object_type( 'post_tag', 'page' );
register_taxonomy_for_object_type( 'category', 'page' );