How to evolve Inkay in Pokémon Go
To evolve Inkay into Malamar in Pokémon Go, you’ll have to do the same thing you did back in Pokémon X and Y: hold your device upside down. Here’s how it works.
This article references relevant content from the polygon.com website. Original article link: [https://www.polygon.com/pokemon-go-guide/22662598/inkay-malamar-evolution-requirements-how-to]

Inkay in Pokémon Go has a special evolution requirement to turn into Malamar. Our Pokémon Go guide will detail how to evolve the Inkay into Malamar, and its candy requirements.
How to evolve Inkay into Malamar in Pokémon Go
To evolve Inkay into Malamar, you need 50 Inkay Candy. At first, the evolve button will be red, meaning you can’t evolve it, but if you hold your mobile device upside down, it should actually flip upside down and turn darker red, as an indicator that you’re doing it right.

If you don’t have a gyroscope in your phone, you should be able to evolve it just by using its candy.
If you’re having a hard time getting your phone to accurately pick up the fact that it’s upside down, toggle any phone orientation lock you may have on or off. You may also need to restart your game by closing out the app completely and re-opening it.

This evolution requirement is the first of its kind in Pokémon Go, but it’s not new to the Pokémon franchise. When Inkay made its debut in Pokémon X and Y, players had to hold their Nintendo 3DS upside down to evolve the little squid Pokémon as well.