Tickets are quantity-based, we only have so many available to us. As tickets are purchased, the price of the tickets may increase.
If the tickets you selected at a certain price point were nearly sold out, and, if someone else were to have completed their purchase before your purchase, you will receive an error message stating that your purchase could not be completed.
Please return to the ticket selection page, and you will see that the price point has gone up or has completely sold out. When you find your tickets and find the right price, proceed to make your purchase directly afterward.