Exodus 16:33-34 (KJV)
33 And Moses said unto Aaron, Take a pot, and put an omer full of manna therein, and lay it up before the LORD, to be kept for your generations. 34 As the LORD commanded Moses, so Aaron laid it up before the Testimony, to be kept.
Here Moses instructs Aaron to store the manna in a golden jar & put it before the Testimony (Ark of the covenant). It seems this instruction was given in the wilderness of sin between Elim & Sinai in the second month after they left Egypt. The Ark of the covenant was erected well after the third month, but it seems the Ark of the covenant had not yet been erected when this instruction was given.
So how could then Aaron have stored it in the Ark of the covenant if was not yet erected?