If you are booking a private transfer, this might happen because you are trying to enter a time after the latest possible pickup time that we have calculated. Please check the message that appears indicating the latest time that you can be picked up at and select that one or an earlier one if you prefer.
If you have booked a shuttle service to the airport, the pickup time will be confirmed to you at least 24h before your transfer departure time. Suntransfers.com will communicate your pickup time via email (and SMS if you requested this option). You can also see the confirmed pickup time by logging in to your 'My Booking' panel 24h prior to the transfer time.