All the museum is 100% complete, and I have a 5 stars town, so, kinda lost here !This happened to my wife's game and we couldn't figure out what it was. Turns out you have to have donated at least one item of each category to the museum. And while you may shrug that off saying you already have done this, you need to make sure you've donated a "sea creature" (i.e. what you have to get a swimsuit and go diving for). There were also dataminers in the Discord saying you need to have at least a 3 star town.
And yeah Xjackio, did all of that already, but Blathers just talked about his posters, and that's it.