Mobile device version:
The Effort Level of a specific Pokémon can only be viewed if the last game it was moved from was Pokémon Legends: Arceus.
If you want to check your Pokémon's Effort Level in Pokémon HOME again, you can do so by moving it back and forth from Pokémon Legends: Arceus one more time.
Nintendo Switch version:
The Effort Level of a specific Pokémon can only be viewed when Pokémon HOME is currently connected to existing Pokémon Legends: Arceus game data.