You can ignore that message since you already have all the available stores built on your island. She also mentioned it to me a couple of times, but I focused on everything else as usual and still got 5 stars in the end.
Considering other 'shops' such as the Roost are basically confirmed at this point, it's most likely dialogue that made it into the game before the actual code of the shops did.
If she wants more storefronts, I wonder if that means that any of the other new stores to be added to the game will affect star rating, making those who have achieved 5 dip back to 4 or even less because they now haven't kept up with everything.