What are Store API errors and why do they break Block checkout?

The Block checkout is powered by the WooCommerce Store API at /wp-json/wc/store. If a security plugin blocks the REST API, permalinks are broken, or an auth/CORS header is wrong, requests like GET /wp-json/wc/store/v1/cart return 401, 404 or malformed JSON, and the checkout stalls. We restore the Store API route, its permissions, and the correct headers so the Block can load the cart and place orders.