So, since The Isle Of Armor came out I brought over my Shiny 100 Chansey from Let’s Go Eevee so I could evolve it into Blissy because I heard you can still evolve a Pokémon if it’s passed Lvl. 100. But since Chansey evolves via friendship and the rare candies said that they have no effect, does that mean my Chansey is going to stay as a Chansey forever? Let me know if there’s another way around this.

I'm fairly certain you're going to need to use the Chansey to raise it's happiness naturally, then use a rare candy. When it "levels up" is when it'll evolve. So it says here! However I've not dealt a happiness evolution in a while, so perhaps it's possible it works differently. But I imagine if you can still raise the happiness in another way it'll still be possible with a rare candy later.

It worked! Thanks so much!