USA and United Kingdom are supported merchant countries, so why are their options absent in the dashboard?
I have got an answer from Klarna support:
The account you requested does not work on the V3 server, hence not on UK & US.
Please see the developer site for UK & US respectively on more info how to set up a test account for the UK, and US.
Another answer from Klarna support:
US/UK is on a separate platform, so your test account does not include these regions.
Please reach out to a salesperson for an eventual agreement in the US/UK and you will be provided with a test account.