“invalid Domain for site key” issue in Captcha in contact us form in Magento 2

Follow the steps to fix “invalid Domain for site key” issue in Magento 2

1. Go to recaptcha admin (https://www.google.com/recaptcha/admin)
2. Choose reCAPTCHA Type (reCAPTCHA V2, Invisible reCAPTCHA or Invisible reCAPTCHA 2).
3. Generate private and public keys.
4. Insert new private and public keys into the Magento site.
5. Then change “reCaptcha type” in the Magento site.

For more details check the link-
Google reCAPTCHA