The error is because of your "=" sign, maybe it should be a "+" or surrounded by quotes.
If it is supposed to be included in the text (ie; You want the your message to be;
":FredS!FredS=@FredS.tmi.twitch.tv PRIVMSG#:The message that I want to send is blah")
Then you need to include quotes around the "=" sign. Change your code to the below;
SendIrcMessage(":" + userName + "!" + userName + "=@" + userName + ".tmi.twitch.tv PRIVMSG #:" + message);
The way you have written it you are trying to assign the value
"@" + userName + ".tmi.twitch.tv PRIVMSG #:" + message
to
":" + userName + "!" + userName
Kind Regards