Problem captcha plugins.

These plugins don’t use standard login and registration hooks, so they don’t work with wpForo. Try this one:
https://www.ads-software.com/plugins/no-captcha-recaptcha/

Checked. This plugin does not work.

Make sure you’ve installed the correct plugin, do you see this menu?
https://screencast.com/t/WcUHnrz1kxT

If it also doesn’t work then I can’t recommend any other.
Also make sure you don’t have other captcha plugins activated, you should have only one active plugin.

This plugin should work fine for registration form but not for login, there is an option to disable it for login form and keep only for registration form.

This plugin runs on a standard registration form. But it does not work on wpForo.

The validation doesn’t go through the standard hooks, or it returns not standard value. We’ll check 3rd party plugin comparability in stabil versions not now.