HELP PLEASE. How does one upgrade from a tent to Nook’s Cranny

My son has been playing AC New Horizons for months now. I have sat down with him at least a half-dozen or so times and read through many tutorials and am at a loss for how to upgrade from the original tent to Nook’s Cranny. He’s paid off his debts. Spent a bunch of money. Upgraded his house. Built the museum. Upgraded his house to a bigger house. Still nothing. I have read there is some prompt that appears and Tom Nook asks for building supplies, but I’ve never seen it. Any ideas?