。
It's not my fault.
Literal
I [subject-が] bad [nominalizer-の] is-not.
Cleft structure: [僕が悪いの] [ではない] = 'the one who is bad is not [me].' Actually, this is a denial of being at fault — 'it's not that I'm the one who's bad.' The の nominalizes 僕が悪い. ではない is the formal negative of だ. A more considered, structured denial than simply 僕じゃない.