、、。・・・。
But we only have one pair of chopsticks. What do we do...?
Literal
But, chopsticks, one-pair [only-しか] not-exist-[explanatory-の]. How do...
膳 is the counter word used specifically for pairs of chopsticks (it also counts servings of rice and meal-trays in classical usage). しか...ない = 'only ~' (obligatorily paired with a negative verb). The final の is the soft, explanatory/feminine sentence-ender. どうしよう is the volitional form of する used as an idiomatic expression of indecision/worry ('what should I do?'). The topic-slot お箸 is marked by a comma rather than a particle — casual topic-drop.