So recipes are usually bought in the exchange store using starlight coins, which you get from arena competitions. You can go to arena -> exchange (piggy bottom left) -> dress -> bathrobe and purchase, or in the crafting screen, you can click the dress itself (there's this red "none" over the dress, which means that you're missing the recipe, and it'll take you directly to the recipe purchase screen)
Side note, if you don't have the starlight coins, I passed that level without the towel (probably got an A or a C? I don't remember) and then went back later, after I had the equinox coat, and got my S. I still don't have the towel since I want to save my stamina to grind the required sets to pass the level (wilds flower was such a pain but the cloud swimsuit one is worseeeee). But you probably want to buy the recipe since it's the last thing you're missing :)
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