Here are some steps you can take:

1. Contact your card-issuing bank to check the status of your card.

2. Try removing and re-adding the card to the app, or use a different card to see if the issue persists. Here's how:

   1. Tap on the Menu button (icon with three lines) in the top left corner of the app

   2. Tap on Remove Card and confirm removal

   3. Tap on Add Card and enter your card details

   4. Try to book another parking session

If the above steps don't work, try closing the app fully and relaunching it. Make sure both your phone's operating system and the Parking.sg app are up to date. If the issue persists, consider deleting the Parking.sg app and reinstalling it to start fresh.