When you arrive at the car park, to enter, wait in front of the entry gate: the license plate reader will register your car and the ticket will be issued automatically. This ticket is already prepaid and has your Parclick reservation information (if you press the button, your reservation will not be valid and you'll be charged the rate price, when your reservation is already paid through Parclick)
When you reach the barrier, the license plate reader will recognise your vehicle and if it does not issue you a ticket automatically, you will have to press the button to collect the ticket and pass through the reception with your Parclick reservation.
Go to the control booth (or ATM) to validate the ticket that you got at the entrance. Location of the customer service booth: on level -1. The pay booths are are next to the lifts where you access the car park. Only in case you have exceeded the time of your booking you will have to cover the difference in price in the car park. Then, drive towards the exit with your vehicle and use this ticket to open the barrier.
Aeropuerto de Barcelona-El Prat (Terminal 1 Llegadas)