It seems like Focus Entertainment no longer supports the patch, but you can still download it from ModDB:
https://www.moddb.com/games/faery-legends-of-avalon/downloads/protection-patch

Extract the files from the archive to the game's directory (C:\Program Files (x86)\Steam\steamapps\common\Faery - Legends of Avalon by default). Confirm the overwrite.

Find the game in your library and open it. Click on Manage (cogwheel icon), then select Properties...

Now select Installed Files and click on Verify integrity of game files.

Steam will automatically repair 1 file. After the process is finished, start the game.

Enjoy playing with achievements!