This may be due to several things:
The purchase process is subject to a checkout timer, so if you do not complete your purchase within 10 minutes, your shopping cart will be emptied automatically
SOLUTION: Try starting the order process again and completing within 10 min.
If your browser (Chrome, Safari, Explorer, etc.) has not been updated for a long time, your purchase may be canceled for security reasons.
SOLUTION: Update your browser and try again - or try to complete the purchase in another browser.