https://github.com/Geta/Klarna/blob/master/src/Klarna.Payments/README.md
https://github.com/Geta/Klarna/blob/master/src/Klarna.Checkout/README.md
https://github.com/Geta/Klarna/blob/master/src/Klarna.OrderManagement/README.md
If you're using Cloudflare/Episerver DXC you need to make sure that the WAF is turned off for the klarnacheckoutapi URLs otherwise Cloudflare might block traffic to it and challenge with a captcha.