The Shiny Charm is a Key Item, only obtainable after completing the Sword and Shield Base Dex, which consists of 400 Pokemon. You don't have to catch all those Pokemon, (you can get them through trading,) but without hacking, you have to have a complete Pokedex to unlock the Shiny Charm.
If you're Masuda Method hunting, your odds are still really good without Shiny Charm, at 1/683. Keep at it, and you'll get your Shiny in no time.
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