This error is shown when the setting "Forbid proxy comment posting" is enabled. If your website passes through a proxy first then it's possible that this is causing a false positive.
You can turn this off by following the instructions below:
Go wp-admin > Settings > Security
Click on the firewall tab on the left side.
Scroll down to "Forbid proxy comment posting" under ".htaccess Features".
Uncheck the checkbox.
Scroll down and click the save settings button.