Can't book a ticket this morning on two browsers. I'm getting the error message:

We were not able to find any available seats meeting your requirements. You may modify your search criteria, or call 1-800-UNITED-1 (1-800-864-8331) within the U.S. or Canada; elsewhere contact your local United Reservations Office.

I check prices almost every day and have had no trouble until today. I purchased a ticket yesterday without issue.