There are several reasons for that
1. Your card might not support Chargybank app. (more in ”What card type can I use?“)
2. You have an unpaid previous order or a fine for not returning the powerbank in time. You either have to put money in the card or add a new card with enough sufficient funds.
3. You have blocked in your bank any transaction from Chargybank app. In this case you cannot use this card for other orders. You will have to add another one.
4. You have insufficient funds on your card.