Plugin won't allow Captcha answer to be entered

If a new user tries to register on my website by email (i.e. using their email address rather than a social login), they are presented with a numerical captcha, but it is impossible for them to resolve it because it won’t let anyone type a reply. When you click in the box to add your reply nothing happens

So, for instance, if the captcha was 4+2 = – it won’t let you enter the number 6. You don’t get an error message, it’s just that nothing happens.

I’ve suspended the plugin for now.

Any suggestions for what might be causing it?

https://ww.wp.xz.cn/plugins/captcha/