OK, I saw you did a search and saw cars. The cars shown based on your search criteria are available. But you only searched for Denia. If you intend to drop off the vehicle in a different location from the pick up location, then tick the box “Return to a different location”. If then no cars are displayed at STEP2, then one way is not permitted between these locations.
For one way rentals sometimes it can be a fee. This is almost always included in the total rental rate and payable at the desk on arrival.