That sounds like a nickname registration problem. If so, you will need to go to the network's #Help channel or other Services help channel and ask them.
From my experience though:
1) Make sure you haven't already AUTHed your nickname
2) Make sure you type the command correctly
3) Make sure the verification code is correct, and you fully copy/pasted it.
4) Make sure the verification code hasn't already expired.
As an FYI, mIRC is one client of many and has nothing to do with IRC services.
Regards,