Can't checkout

Works fine until I go to checkout. Then I get a "Site Error" message followed by "

We're sorry, the link that you clicked on could not be served. We are experiencing a temporary delay due to high volume. Please click RELOAD and try again. We apologize for the inconvenience. "

This has been happening now for 3 weeks. I have an order in my shopping cart waiting to place since June 17th it's now July 9th and I still can't complete this online order.