you need to finish the musem to get the rusty key so u can meet kroubus and kroubus gives u a talismanYou don’t need to finish the museum, but you need to donate enough objects to get the key to the sewers. I think it’s 60 (or 65?).
No, you don't, please have a look here: https://stardewvalleywiki.com/Rusty_Keyyou need to finish the musem to get the rusty key so u can meet kroubus and kroubus gives u a talisman