The documentation for the «allowed_customer_types» order request field says:
List of the allowed customer types.
Allowed values are ‘person’ and ‘organization’ if B2B is enabled on the e-store ID.
https://developers.klarna.com/en/se/kco-v2/checkout-api#options-object-properties
So how to enable «B2B» for a Checkout v2 merchant account?