Some users find Google Checkout times out or crashes on their phone when they try to add a new credit card. This is a bug with the Google Play Store.
You can resolve this by using a laptop or PC to access Google Checkout, adding your card there.
When you return to your phone, the card should be registered and you should then be able to purchase SwiftKey without complication.
If you continue to have trouble, please contact Google Play Store Support here: https://support.google.com/googleplay/?rd=1#contact=1