Great question! At this time, debit cards are not a supported payment method for the Shopify monthly subscription. It will require using a credit card issued by a bank with a line of credit. Debit cards with credit card capabilities are not supported. If you do not have a credit card, then applying for one with your bank would be the best route to take. If you're operating outside of the United States, then you can also pay for the Shopify subscription with Paypal.
I'm afraid a credit card would be the only option if you are based in the United States. If you are based outside of the United States, then you could use Paypal. You may want to consider asking a friend or family member for their credit card to process the monthly charge and pay them instead as that is also a viable workaround.