There's no function findPlayerByName, unless you create one. If you need to get player with a specified nickname, use getPlayerFromName. And it's better to store function result in a variable if you need to use it a few times. I'll try to correct your code
addCommandHandler("use",
function(player,command,Fun,FunPlayerName)
local playerToFun = getPlayerFromName(FunPlayerName)
if playerToFun then
outputChatBox (FunPlayerName.." #FF0000is use "..Fun.." with "..getPlayerName(player),getRootElement(),51,255,0,true)
end
end
)