I am trying to log in using Opera Mini.
How can I solve the 'Verification Human CAPTCHA Secret sign', since it doesn't pop up any button to click on?
Opera mini is a robot, since it downloads the page from the server and renders an altered version of the page which is sent to your device. Thus it lacks being able to login, retain cookies, etc.
From the linked Wikipedia page:
For this reason, Opera mini is not supported as browser for anything other than viewing.