The real-time ticketing system is currently operating at its full
capacity of simultaneous customers. A limit is placed on the number of
people simultaneously within the ordering system to ensure a smooth
operation at peak periods. You will therefore need to retry a few times
until a spare slot becomes free as other people finish the ordering process.
These extremely busy periods normally occur in the first few hours of a new
event going on sale, so alternatively try again a little later when the
initial rush will have died down.