when you use it on the statue, does the statue change color? Are you using it on the statue after day 27 but before Scarlet asks for an expensive item?
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I'm having the same issue, using the slime or any other item when she asks is impossible. You aren't blocked from giving her a potion though, but on later adventurers her text shows up on them and when you reach stacy blowing gum she also has the crimson blade text and kills you giving the same hint to use the slime on the statue. You also can't use the slime on the statue at this point.