[Patrick]

Here you go, I've included some explanation inside, Enjoy!

pawn Код:

public OnPlayerCommandPerformed(playerid, cmdtext[], success)//callback from #include <zcmd>
{//open
    if(!success)//If the command does not exist on the server, ! = false
        return SendClientMessage(playerid, COLOR_WHITE, "Your Error Message Here.");//showing the message
    return 1;
}//close