The given fields in the docs seem to differ from those that checkoutCompleteWithTokenizedPaymentV3 really wants from us. This is the response I get whenever I call the mutation. I don't see these fields in the docs. Feel free to correct me if I misunderstood something,
By the looks of it your checkout object was missing those fields. They may not be required in the completion mutation - but they are required to submit. From my first glance it looks like you are missing the steps on the guide here: https://shopify.dev/tutorials/create-a-checkout-with-storefront-api for associating an email, shipping address and shipping rate. These steps are covered in the above guide.