Sofort is good for paying money, but a completely flawed method to get the IBAN from someone in order to later do a direct debit. I'm curious if that's actually allowed by Klarna in their terms of service. After all, they also lose out on some transaction fees I'd reckon.
Actually now that I think about it, the website https://www.telekomaufladen.de also requires a Sofort payment of 10 cents to set up a direct debit. I set it up a few weeks ago but as of now I still have enough money on that SIM card so I never used it. I'll see in a few days when I have to renew that SIM's plan for another month if it works or not.
Maybe the support can help you to manually set your IBAN, or you could ask them if you can use iDeal as a German customer.