The flights are operating on those dates with plenty of seats to sell.
If you have multiple names, make sure you are typing them into the form the same way as they are shown on your confirmation.

EDIT: Two other thoughts - are you trying to maybe check in rather than checking the status of your booking using "manage my booking?" Were there any changes to the reservation? If so, that could trigger that message as well, and you may need to call the airline.

Their US number is 1 800-344-6726