Shiny Pokemon are Pokemon that are of a different color than normally found, being extremely rare to find in the Wild. However, Shiny Pokemon are no stronger than a regular Pokemon. Every Pokemon has a Shiny Form, but not all Pokemon can legitimately be found as a Shiny Pokemon. Pokemon that cannot be found in their Shiny Form are known as Shiny Locked Pokemon.
Most Shiny Locked Pokemon are either Cover Legendary Pokemon or Mythical Pokemon that can only be obtain by an event. At the release of Pokemon Sword & Shield, only ten Pokemon were Shiny Locked: Victini, Keldeo, Meloetta, Hoopa, Volcanion, Cosmog, Cosmoem, Magearna, Marshadow, and Zeraora.
Now, that number has increased to thirteen Shiny Locked Pokemon. That's right, all but three 8th Generation Pokemon can be found as a Shiny Pokemon! That said, it comes as no surprise that the three Shiny Locked 8th Generation Pokemon are the Legendary Trio (Zacian, Zamazenta, and Eternatus). Even though these three were coded as Shiny Locked, that doesn't mean they (as well as the other ten Legacy Shiny Locked Pokemon) won't one day become available in their Shiny Form.
We have questions and answers related to this topic which may also help you: