Sorry that you're having this issue placing your order. Can we ask you to perhaps try a few things, which might fix your problem.
1. Can you try and reset your shopping cart by clearing all your web browsers cookies
2. Closing your browser and try starting again on the website. (This creates a new shopping cart)
If you are still experiencing issues, then please try on a different device or contact your issuing bank as they will be able to help further.