The klarna checkout plugin isn't supported with Robot Ninja out of the box.

Plugin url: https://woocommerce.com/products/klarna-checkout/

Some notes:

- checkout uses a different page to standard wc checkout page and ignores whatever is set for the checkout page in settings

- has custom checkout billing field elements and custom payment methods

- payment is a two step process or atleast payment happens off-site or on the second step