。
She learned to ride a bicycle last year.
Literal
She [topic-は] last-year bicycle [on-に] can-ride became.
乗れる is the potential of 乗る ('to ride'), and ようになる closes the construction with 'came to do / came to be able to.' This is a workhorse pattern for newly acquired abilities or habits: 食べられるようになった ('came to be able to eat'), 行くようになった ('started going'). The pattern emphasizes the change-of-state — the gradual settling-in of the new ability — rather than the moment of learning, so it's a natural fit for skills that develop over time like riding, swimming, or speaking a new language. Note that 乗る takes its vehicle with に, not を, treating it as the surface one boards onto.