Starting from wpDiscuz 7.0.0 you can use Google reCAPTCHA version 2 "I'm not a robot" antispam protection. In some cases, depended on your server PHP configuration it may not work and return  "reCaptcha validation fails. Error code: invalid-json" error.

If you see this error, please navigate to Dashboard > wpDiscuz > Settings > Google reCAPTCHA admin page, find the "Request method" option and change the value of the option to "CurlPost", delete all caches and check again. If it doesn't help select the values one by one to find the request method that will work fine in your case.

Press CTRL+F5 (twice) on the frontend each time before checking.