I tried reproducing the error by adding whitespace to the username, in case that freaked it out.
After seeing the comma in my attempt, it occurred to me: in your screenshot, it's trying to send a PM to a user called "@evandixon" instead of "evandixon". "@evandixon" doesn't exist, so "@evandixon" can't receive messages.
I blame the vague error message for the confusion. If only it said "@evandixon isn't a user" or something.