Yes, Hosts can require Guests to pay a fully refundable security deposit.
While we cannot process security deposits as payment "Extras" for bookings, you are free to ask your Guests to provide a security deposit in cash or by using other payment methods (e.g. check, Venmo, Paypal). You can collect the payment however you want but you must guarantee your Guests that you will refund the entire amount at the end of the booking, provided there are no damages to the rented space.
If you want to require a security deposit, we recommend to be specific. Do not forget to include the information in your listing details by including the amount of money required for it and how/when it will be collected and refunded.