You've tried to confirm a booking and you've received the following message: "The owner's vehicle is already booked for this time"?
This could mean that the owner has perhaps blocked their calendar to keep it available for you. Please make sure to contact them to let them know this is keeping you from paying and that they must unblock their calendar for the dates chosen, and try again.
If, however, their vehicle has in the meantime been booked by another renter, you will be informed by email. You will then be able to send other requests for new vehicles.