Without the code of your Custom Post Type it’s kind of difficult to answer. I have few hypotheses :
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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.
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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.
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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
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Please make this change, save your functions.php file and flush your Permalinks (step 1) right after it.
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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 !