。
The question is who made the mistake.
Literal
Problem [topic-は] who [subj-が] that mistake [obj-を] did [embedded-Q-か] [という thing-である].
失敗をする ('make a mistake, fail') is the standard collocation for 'make a mistake.' 失敗 as an abstract noun is treated as something one 'does' (する) rather than 'makes' (作る in English style). A typical Japanese noun+する structure.