Debit Card Number vs Bank Account Number
There are two separate ways to pay online. You can use a “card number,” or you can use a “bank account number.” RentHelper does not take card numbers. RentHelper only works with bank account numbers.
Your bank account may come with a debit card. In this case, you have both numbers. RentHelper does not work with the card, it works only with the account number.
Card Numbers Not Accepted

Card numbers do not work. Image CC BY-SA jackbtcjack123 wikimedia
Most card numbers are 15 or 16 digits. If you have a number that has 15 or 16 digits, it is probably a card number.
Bank Account Numbers Accepted

You can find your account number on a bank statement, via online banking, or by calling the number on the bank of your card.
Most bank account numbers are shorter than 15 or 16 digits.