Is the nick that you're using registered with nickserv? You can find out by doing a
If it is registered, and you're the one that registered the nick, then identify with
/ns identify YourPassword
If the nick is registered, and you're not the one that registered the nick, then you must choose a different nickname, that isn't registered, and register it. This is all assuming that the network requires you to be registered and have usermode +r set before you can receieve any files.