Clear the cache by deleting the folder at C:\Users\[Username]\AppData\Local\Ubisoft Game Launcher .
If the file is truly missing, use the launcher to repair it:
How to Fix "uplay_r2_loader64.dll Missing" in Assassin's Creed Valhalla
If you own a legal copy of Assassin's Creed Valhalla , follow these steps in order.