Hi!
When you join a new channel, you're automatically a channel operator, op, (@ prefix).
But if someone is already there, op or not, you can't until someone ops you.
If you try to get op using ChanServ from an unregistered channel, it's logic that you get the error notice "channel not registered".
On most networks, you need 1st to register your nickname (see NickServ help register)
Then, join a new and unique channel where nobody is, and now you can register you channel using ChanServ register.