Invalid site key

That error has nothing to do with Really Simple CAPTCHA.

Hi @arammelsberg,
You assigned Google reCaptcha for your website and need to get site key from google reCaptcha page. Your Captcha option actually work form Google I think. Check that.

Contact Form 7 now uses Google’s reCapthcha v2. If you go to the “integrations” tab, you’ll find a link to take you to the right page at google.

Thank you so much @sterndata! That worked!