From FrontierAirlines.com
online redemption maintenance information
To enhance your experience, modifications are currently being made to our Online Redemption booking option. As a result, you will be unable to book award travel online from Monday, February 26 - through Thursday, March 1st. If you need to book award travel during this time period, please call Reservations at 800-4321-FLY (800-432-1359).
We apologize for any inconvenience.
Also, you are now able to redeem EarlyReturns miles for Frontier award tickets at the airport effective immediately...have your CSA refer to alert 07-14.