Your card must be open for online and international purchases, allow recurring payments, be valid for more than six months forward and there must be at least 1kr/1€ for a verification to go through. The card needs to be a Mastercard, Visa or American Express issued in Europe.
If you still can't register your card start by ensuring you're connected to the internet, either through mobile data or WiFi. You can also try restarting your phone or reinstalling the app.