So Marussia have a reserve driver that is not able to drive the car!
Maria de Villota is not Marussia’s reserve driver. She never was. She is officially designated as their test driver. Because of this, Marussia do not actually have a reserve driver, which is entirely their choice and hardly a mark of incompetence – Sauber did not have a reserve driver in 2010, for instance.
so why weren’t they allowed to replace him?
In order to take part in the race, you must first qualify the car. Glock sat out qualifying in the hopes that he would be fit for the race. He wasn’t, so Marussia were unable to put anyone in the car for the race.