It occurs when you use triggerClientEvent on a player that isn't finished downloading or loading it's scripts. In most cases it can be ignored. In your case it means that your zombie isn't muted for a specific player.
If you want to optimise it, you should keep track of loaded players and only send messages to that segment. And also send a message to new loaded players.
It is a time consuming fix, unless it was correctly done from the beginning.
Yes, I have already made this definition. Thanks. But after the fix, another error occurred: server triggered clientside event Zomb_STFU, but event is not added clientside. Although I seem to have added an event:
addEvent( "Zomb_STFU", true )
function Zstfu ( ped )
if (isElement(ped)) then
addEventHandler( "Zomb_STFU", getRootElement(), Zstfu )