Not every Pokémon in Pokémon Go has only a candy requirement to reach before you can evolve it into its next form. Some Pokémon require evolutionary items, such as evolving Scyther into Scizor, you need to have a metal coat item in your inventory. You receive those at PokéStops, and it’s typically random about which of the several items you receive. For Galarian Yamask, you need to complete a quest to evolve it into Runerigus.
For your Galarian Yamask to evolve, you need 50 Yamask candy and to complete 10 raids. These can be a raid of any difficulty, such as completing a one-star, three-star, five-star, or a Mega Raid. The five-star and Mega Raids contain some of the strongest Pokémon for you to face in Pokémon Go, so you if you purely want to evolve your Galarian Yamask, you’re better off finding a one-star raid to battle against or take a friend with you to fight in a three-star encounter. These are some of the easier options, and you can usually find them around most cities.
A raid can appear at any of the Gyms in-game. These are usually massive PokéStop-like structures that are controlled by one of the three Pokémon Go teams. If your team controls the gym, it does provide you with additional rewards after you complete it.
You do need to win to complete a raid battle. Once you’ve completed 10 of these, Galarian Yamask will evolve into Runerigus. Many trainers might find this task annoying because everyone receives a certain amount of raid passes each day. Unless you want to purchase them using real-world money or Pokémon Go’s premium currency PokéCoins, you likely need to wait to receive more before completing all raids for this quest.