There can be several reasons why your register button is not working,
1) There is an error somewhere in your registration form, generally this may be that an email address has been misspelled and therefore doesn't match. Please check the form and look for any highlighted red text that will alert you to the issue
2) You have ticked "I am not a robot" but have failed to tick the "Terms & Conditions" box
3) The "I am not a robot" tick is being blocked by your browser. In older browsers, the robot box will generate some images to verify you. These can be blocked and therefore I'm not a robot will not complete. You can, if you aren't already, try registering on a newer browser such as Google Chrome or Microsoft Edge.
If, after checking all of these options, you are still unable to press the register button, please email firstname.lastname@example.org with screenshots of your registration from.