When attempting a purchase through the Steam Store, I am getting the following exact error message:
How can I complete my purchase?
If the specific error message that you receive is one of the following, click the link to the correct support page for that issue.
This error can occur when our system has encountered an issue with the transaction that you are attempting to make.
Test the issue once or twice more, ensuring that you are entering the correct billing information. Avoid spamming failed purchase attempts more than a few times.
If you continue to encounter this error, please use the Help site to select your failed purchase and contact Steam Support for further support.
Disable or completely remove any IP proxy or VPN software that you may be using, restart your computer, and test your purchase again. All transactions that are initiated through anonymous proxies will be declined by Steam.