PayPal Donations gateway requires you to connect in both live mode and test mode if you want to fully test without using real money. If a user only connects to one or the other (live mode connected to a live account or test mode connected to a sandbox account) but then views a form in the other mode, credit card fields are displayed but don't actually function.
When that's the case, instead of displaying essentially dummy fields GiveWP should alert the user to connect in the other method to PayPal.