The error message you are seeing indicates that the HTTP response version being sent back from the payment gateway is invalid. This could be caused by a few different things.
One possibility is that your server is using an outdated version of cURL, the library that handles HTTPS requests. Magento 1.8 was released in 2013 and may not be compatible with the latest version of cURL, which supports HTTP/2. You could try updating cURL to see if that resolves the issue.
Another possibility is that the payment gateway is using HTTP/2 to send the response, but your server is not configured to handle HTTP/2 requests. This could be a problem with your web server, such as Apache or Nginx, or with the PHP version that Magento is using. You may need to update your web server or PHP configuration to support HTTP/2.
If you are not comfortable making these changes yourself, you may want to consider hiring a developer with experience in Magento and server administration to help troubleshoot the issue. They will be able to review your server logs and configuration to determine the root cause of the problem and recommend a solution.