Weather plays a vital role in Pokémon Sword and Shield because particular Pokémon only show up in regions based on the weather conditions. For example, you’re only going to find a Mimikuy when it’s foggy out. There are ways to change the weather, but several players have run into the issue of not being able to get the foggy weather to show up.
Having the Fog weather show up is quite rare. The sure-fire method to have Fog start appearing in the game is to beat the game. You need to progress through the end of the story. However, some players have reported they’ve encountered Fog earlier. While it can happen, rarely, the Fog weather starts to become much more common after beating the game.
You’ll know when you’ve beaten the game when you see the credits roll. This weather doesn’t happen when you get all of the Gym Badges because there’s still more for you to do.
After you’ve completed the story, you can modify your Switch’s time settings to have Fog show up more often reliably. You need to make sure you set your Switch’s date to 6/1/2020 to have it spawn.