[Konstantinos]

It should be the opposite.

pawn Код:

if(arroz == INVALID_VEHICLE_ID)
{
    RemovePlayerFromVehicle(playerid);
}
else
{
    SendClientMessage(playerid, -1, "This is your car");
}

It's supposed you assign to arroz INVALID_VEHICLE_ID.

If the vehicleid matches with the vehicle's ID you entered, then it will change and it will no longer be INVALID_VEHICLE_ID.