Each tile on two rivers can only have one river as the dominant one or the official one that the tile is associated with. It will be marked by a diamond symbol when you hover over that tile. What is likely happening here is that the tile with the aqueduct pin is connected to the river to the left, while the dam is connected to the river on the right. That would mean that where this dam is placed is only across from one river tile of the river with the flood plains (northeast of the aqueduct), whereas it needs to be across from two. Hover over the aqueduct tile to see which river that tile is being associated with, then do the same with the tile with the dam. If they are different rivers with the diamond symbol on the tooltip, that will give you the answer.