It won't be closed, I suspect you're hitting the 10 day booking length restriction. I've found before that Heathrow doesn't permit bookings in the Pod Parking longer than 9 days 23 hours. If you shorten your search you'll see it suddenly become available.

Website doesn't seem to distinguish between full and restricted, hence it just tells you "Sold Out". Also be aware that Heathrow includes an extra half day on top of whatever you search (cover for late flights), so its the total which must be under 10 days.