[iggy1]

pawn Код:

if(IsPlayerInVehicle(playerid, 420 && 431))

You do know a player cant be in two vehicles at the same time?
make that

pawn Код:

if(IsPlayerInVehicle(playerid, 431)||IsPlayerInVehicle(playerid, 420))

EDIT: Fix

pawn Код:

public OnPlayerCommandText(playerid, cmdtext[])
{
    if(strcmp(cmdtext, "/MyCommand", true) == 0)
    {
        if(IsPlayerInVehicle(playerid, 431)||IsPlayerInVehicle(playerid, 420))
        {
            new vehicleid = GetPlayerVehicleID(playerid);
            if(IsPlayerInAnyVehicle(playerid)) SendClientMessage(playerid,0x00FF00AA,"You're not in a vehicle.");
            SendClientMessage(playerid,0xFFFFFFFFF,"Your veh have been tuned!");
            ChangeVehicleColor(vehicleid, 103, 103);
            AddVehicleComponent(vehicleid,1010); //Nitro
            AddVehicleComponent(vehicleid,1001); //Spoiler-Win
            AddVehicleComponent(vehicleid,1076); //Wheels-Wires
            AddVehicleComponent(vehicleid,1012); //Hood-Worx-Scoop
        }
        return 1;
    }
    return 0;
}