This error comes from the UsersWP ReCaptcha add-on when Google does not confirm the captcha. Either the user did not complete it, or the check with Google failed. When it shows even after ticking the box, or the submit button stays disabled, the cause is almost always the keys or the domain.

Check the keys

1. Go to UsersWP > Addons > ReCaptcha and note which reCAPTCHA type is selected.
2. In the Google reCAPTCHA admin console, open your site and check its type. A key pair made for v2 “I’m not a robot” only works with the v2 checkbox, invisible keys only with invisible, and v3 keys only with v3.
3. Copy the Site Key and Secret Key again from Google and paste them into the add-on settings. Watch for spaces at the start or end.
4. Save and test in a private browser window.

Check the domain

Google only accepts the captcha on domains listed for that key. Add every domain you test on, including your staging domain. For local development, add localhost to the domain list, or create a separate key for local use.

Other causes

• Make sure the reCAPTCHA plugin from another vendor is not also active. Two captcha plugins on one form conflict.
• Exclude the register and login pages from page caching and JavaScript optimisation. A cached page can carry an expired token.
• Firewalls and security plugins can block the request from your server to Google. Ask your host if outgoing requests to google.com are allowed.

Setup steps are in the ReCaptcha setup guide and the settings are explained in Settings overview for UsersWP > Addons > ReCaptcha.