First you should confirm you have checked your system is compatible with our system requirements, which are outlined in the Simulation Player Guide and also separately in our Knowledge Base. If they appear to be compatible, and you still can't submit your payment, the most likely issue is your Internet connection has dropped out temporarily. You need to have a stable Internet connection to complete payment and to play the simulations.
If you have checked your Internet connection and are sure you have a stable connection another issue could be the credit card you are using. If you have another credit card you could try that to see if that solves your issue.
If you still are experiencing a problem please submit a support request with detailed information about your operating system and browser, confirming what steps you have taken already to troubleshoot the issue, and if possible supply screenshots.