Versuche es so:
ocmd:plate(playerid,params[])
{
new text;
new fahrzeugid = GetPlayerVehicleID(playerid);
if(sscanf(params,"s",text))return SendClientMessage(playerid,ROT,"Benutze: /plate [Nummernschild Name]");
SetVehicleNumberPlate(fahrzeugid,text);
return 1;
}