I kind of think it's unreasonable of a guest who doesn't have either a credit or debit card (or a Paypal account, or similar online methods of payment) to expect to be served by an online booking platform, @William554.
My suggestion to this type of guest would be to find a more "bricks and mortar" solution to their accommodation needs (ie. a traditional BNB or a hotel that accepts cash payments {if they exist}).