At the moment you cannot transfer the outstanding amount directly. There are two options to help you:

Option 1: depending on your bank, up to three more attempts will be made to debit the amount from your account. Please note: you cannot check in and out again until the outstanding amount has been paid.
Option 2: do you want to travel right away? Make sure you have enough balance on your account. Hold your card against a card reader again. The debit is then reactivated and debited if your account has sufficient balance. If this went well, you can check in again with your debit card after a maximum of 15 minutes.