。
She blamed her failure on her illness.
Literal
She [topic-は] her own failure [object-を] illness's blame [as-に] did.
~のせいにする ('to blame X on Y, to attribute X to Y as the cause') uses the productive [X]を[Y]にする pattern with せい ('blame, fault') as Y. ~のせい always carries a negative connotation, attributing an unwanted outcome — distinct from ~のおかげ ('thanks to'), which credits a positive one. So 病気のせいにする has a built-in nuance of deflection or scapegoating, even if the attribution is factually correct.