55 This anthropomorphic expression concerns the power of God. The “hand of the Lord” is idiomatic for his power, what he is able to do. The question is rhetorical; it is affirming that his hand is not shortened, i.e., that his power is not limited. Moses should have known this, and so this is a rebuke for him at this point. God had provided the manna, among all the other powerful acts they had witnessed. Meat would be no problem. But the lack of faith by the people was infectious.
56 Or “will happen” (); “shall come to pass unto thee.”

In other versions

And the Lord said to Moses, “Is the Lord’s hand shortened? Now you shall see whether my word will come true for you or not.”

The Lord answered Moses, “Is the Lord’s arm too short? Now you will see whether or not what I say will come true for you.”

The Lord answered Moses, “Is the Lord’s arm weak? Now you will see whether or not what I have promised will happen to you.”

Then the Lord said to Moses, “Has my arm lost its power? Now you will see whether or not my word comes true!”

And Yahweh said to Moses, “Is Yahweh’s power limited? Now you will see if my word will happen or not.”

And the Lord said unto Moses, Is the Lord’s hand waxed short? thou shalt see now whether my word shall come to pass unto thee or not.

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