。
She still can't ride a bike.
Literal
[topic-は] still bicycle [on-に] cannot-ride.
自転車に乗る ('ride a bicycle') uses the に-pattern for vehicles — 乗る takes に for the conveyance one boards. The negative potential 乗れない ('cannot ride') marks the lack of skill rather than circumstantial inability — for the latter the structure 乗ることができない would be more typical. Cultural detail: bicycles are everyday transportation in Japan, especially for elementary school commutes, shopping, and short urban trips. Many Japanese children learn to ride at a young age, so 'still can't ride a bike' implies missing what's normally a basic milestone.