[Jeffry]

pawn Код:

public OnPlayerCommandText(playerid, cmdtext[])
{
    new Text[100];
    format(Text, sizeof Text,"[Error] Command /%s don't exist!", cmdtext);
    /*Your commands here
    here...
    here...
    blah blah*/

    return SendClientMessage(playerid, COLOR_ERROR, Text);
}

Should work then.

EDIT: MyLife was a bit faster.