Every form submission is protected from Cross-Site Request Forgery (CSRF) by a security check. This error means that the security check has failed. The security check relies on the proper functioning of cookies in the browser and the plugin session system.
If you are using a browser extension, add-on or app that could be blocking cookies in the browser, try temporarily deactivating it and test the form submission again.
You can disable the security check at Forms → Settings → Tweaks & Troubleshooting, turn off the option CSRF protection and save the settings. It is not an essential security feature – your site will not be at risk if you turn off this option.
Note that this soltuion will just prevent the error message from being displayed, it will not fix the underlying cause of the problem. You may still have issues with the File Upload field and the CAPTCHA field, which both also rely on the proper functioning of cookies in the browser and the plugin session system.