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.