You need to configure your site keys adding your public and private keys in the
UmbracoForms.config file located in
<setting key="RecaptchaPublicKey" value="sHZZenninFziVUV9TN24FqhwZvc2b4e8BLrG" /> <setting key="RecaptchaPrivateKey" value="sHZZenninFziVUV9TN24FqhwZvc2b4e8BLrG-" />
You can create your keys by logging into your reCAPTCHA account.
Note: Don't forget to make the Recaptcha field mandatory.