The reasons why a credit card is always essential
To protect themselves, the rental agency must be sure that any damage to the vehicle can be reimbursed by the customer, up to the amount indicated by the excess (not to be confused with the deposit!). The management of these sums is done exclusively by credit card. In fact, debit or prepaid cards, cash deposits or checks are not accepted.
The agencies block the amount of the security deposit on the card which, working on a "credit" basis, will guarantee the rental agent the possibility of freezing and possibly charging an amount even higher than that physically present on the account, fully protecting itself from accidents or thefts.
This amount will eventually be defrosted when the vehicle is returned, if the car has not been damaged. Of course you can discover the maximum amount that can eventually be withdrawn from the card (the excess) by reading the terms and conditions of each vehicle.
Debit or rechargeable cards do not constitute a guarantee for the security deposit: this is because they depend exclusively on the money that is present in the current account when the car is rented.