Mobile Device Version
Your Pokémon’s Sheen and condition can only be viewed if the last game that the Pokémon was moved from was Pokémon Brilliant Diamond or Pokémon Shining Pearl.
If you want to check your Pokémon’s Sheen or condition in Pokémon HOME, you can do so by moving the Pokémon back and forth from Pokémon Brilliant Diamond or Pokémon Shining Pearl.
Nintendo Switch Version
The Sheen and condition of a specific Pokémon can only be viewed while Pokémon HOME is connected to a linked Pokémon Brilliant Diamond or Pokémon Shining Pearl game.