。
It doesn't reach the skylight.
Literal
Skylight [to-に] not-reach.
明かり窓 (あかりまど) is a skylight, transom, or dormer window — literally 'light window.' 届く ('to reach, arrive at') takes に for its destination. A short, enigmatic sentence — without context, we don't know what doesn't reach (light? voice? an object?).