Taxonomy Archive URL + Template

Without the code of your Custom Post Type it’s kind of difficult to answer. I have few hypotheses :

  1. Have you flushed the permalinks ? You have to go to Settings > Permalinks > and click on Save to do so. Repeat this action anytime you modify the rewrite rules of your Custom Post Type.

  2. Is the Taxonomy slug the same than the Custom Post Type slug ? If so, I guess this is why you have a 404 error page.

  3. In your Custom Post Type and in you Taxonomy Post Type, if you want a specific url (and not the default one) as a slug, you can use this argument (please this documentation for more details) :

    ‘rewrite’ => array( ‘slug’ => ‘you-slu’ ), // you can replace you-slug by your custom slug

  4. Please make this change, save your functions.php file and flush your Permalinks (step 1) right after it.

  5. The template for displaying your taxonomy page will be taxonomy-[slug].php (you have to replace slug by the slug of your taxonomy).

Hope it helps !