If you have an issue with using API credentials you’ve generated, start trouble-shooting here:

Make sure the test mode is disabled
Try copy-pasting credentials again (check there are no typos or extra spaces)
Verify that you’re using the correct integration. We offer two integrations – full checkout replacement, and Klarna as a payment option within your existing checkout solution. Your credentials will only work for the integration you’ve signed up for
Make sure you’re not using