。
She was too scared to go through the forest.
Literal
She [topic-は] scared [so-て], forest [path-を] was-not-able-to-pass-through.
The を on 森 doesn't mark a direct object — 通る is intransitive — but instead marks the path traversed, the same を used in 道を歩く ('walk down the road') or 橋を渡る ('cross the bridge'). When this path-を verb is put into potential form (通れる, 'can pass through'), the path-marker stays as を; only direct objects of transitive verbs flip to が in the potential. Forests are stock settings in Japanese folktales for places of fear and danger — the haunt of foxes, tengu, and yamauba — so 森を通れない has a quietly fairytale-ish ring even outside that context.