Here you go, mate, try this...
https://mybadstudios.itch.io/simple-achievements
All you need is a unique name for each user and you are good to go. You can create your game's achievements right inside of Unity using the included prefab and for the achievement's images you can either store a URL then fetch the graphics online or you simply save the name of an asset in a Resources folder and you are now ready to move on to the rest of your game.