@Sadeeem Clicking the link in the email? If you've clicked the link in the email sent to you after signing up, have your tried signing in to auth.opera.com and confirming that your Neon subscription is there? I am sorry for the lateness of my reply but I am actually part of the community team and not the support team. If we can't solve this now, I'm going to have to ask you to email neon@support.opera.com tomorrow.
But I know that the process is:
You sign up. You pay for the subscription. You register an Opera account. You click the activation link in your email. You download the Opera Neon app from a link in the email or from a download link on our auth.opera.com on your Neon subscription page.Now Neon should be active and you should be logged in. I must confess I haven't seen the specific error message that you're displaying so I'm going to log this as a bug/error with the support org. Hopefully they will get to you a bit earlier than the regular support team.
I'm really sorry you've run into this problem.