Are you doing this on a twitch server? It's acting like WHOIS is a command not recognized by the server, which shouldn't be true on a 'real' irc server, but i've been on twitch enough to know that they don't follow the rules. If /w is a twitch command like 'whisper', then you need to edit the Alt+D aliases window to remove the 'w' alias there or rename it to something else like 'ww' instead.
Also, for most 'real' servers, the @ is a prefix indicating they're a channel OP, and their real nick excludes that symbol. But maybe things are different at twitch. Reminder for everyone else, if you're asking a question related to twitch, mention that in your question, and you avoid answers from people who don't know what they're talking about, such as me.