。
She can't ride a bicycle, let alone a motorcycle.
Literal
She [topic-は] bicycle [contrast-にも-にはもちろん], motorcycle [also-にも] cannot-ride.
The same ~はもちろん frame as in the affirmative 'X goes without saying, Y too,' but here under negation. The logic flips: bicycle becomes the obvious thing she can't ride (the easier baseline), and motorcycle is the further extension also out of reach. So the natural English is 'can't ride a bicycle, let alone a motorcycle' — Japanese marks the easier item with もちろん, English marks the harder item with 'let alone.' The composite particle にも (target に + additive も) compounds 'on a motorcycle as well.'