Complete the captcha before continuing
Registration (or a shared captcha gate) refused submit because the challenge was empty, expired, or not finished.
Why this happens
Captcha is configured and shown; prepareSubmit failed. Distinct from missing-keys misconfig. Login often uses the shorter Complete the captcha to continue wording.
Diagnose and fix
1. Complete the captcha, then submit again.
2. Allow captcha scripts (ad blockers often break the widget).
3. Admin: confirm keys and Captcha on registration.
4. If verification fails after solving, see Captcha verification failed.