Sorry, no postcard matches your number. You may have mistyped the numbers, or your postcard may have been already picked up. Please take into consideration the fact that postcards are stored only for 14-30 days after the sent date.
Try again, or if it still does not work and you are curious about what your postcard looked like - go back to the Post Office and send one to yourself (just to make sure it works-:)
Sometimes you can see this error message if you added spaces before or after the card number. Remove those spaces and very probably you will get your card.