Helen here from Shopify Support. I’m here to help!
Which payment gateway are you using? How are you attempting to place a test order?
One reason you may be getting that error message is if you are trying to process a real transaction while your payment gateway is in test mode.
To see if your gateway is in test mode, please go to ‘ Settings > Payment Providers’, and notice if there is a yellow banner above your payment gateway on that page, like below:
However, if you are wanting to proceed with a real transaction with it, or any other gateway that is integrated, then please click ‘Turn off test mode’ in the banner, scroll to the bottom of the page and uncheck the ‘Enable test mode’ option as seen here:
Let me know if any of these options help. Otherwise, I am more than happy to continue working through this with you to be sure your gateway is functioning properly.