I disabled option Enable Captcha Image For Registered Users and now it's ok.
What is Enable Captcha Image For Registered Users?
Is it related to Google ReCaptcha?
Why it's not working when enabled?

This is the second type of captcha (screenshot: https://monosnap.com/file/MLLSTia9ODfehfToHARIQu3V5gsh15 ) which you need to manually type the captcha validation key before you submit, and it is totally different than Google Recaptcha type, the system only allows you to select one type of captcha from the backend.

And I noticed that it doesn't auto-reload the captcha validation key if the validation failed, I will log this issue into our issue tracker.

And finally, I have set Skip Captcha after Certain Post Count = 1, and I have several posts, so captcha should not uses at all.

If you set "Skip Captcha after Certain Post Count" to 2, meaning the user needs to publish at least 2 discussion posts and above then the system only will skip the captcha validation during submit a new post.

From what I tested locally, it seems like the calculation logic is wrong, if you set "Skip Captcha after Certain Post Count" to 2, the user needs to submit 3 discussion posts in order to skip the captcha validation, so I will log this issue into our issue tracker as well.

Not clear, what exact should I do now.

And finally, I have set Skip Captcha after Certain Post Count = 1, and I have several posts, so captcha should not uses at all.

Are you referring to this issue?

If so, may I know which discussion post can we test on it? Or can I create a test discussion post on your site to reproduce this issue?