If you purchased Fallout 76 digitally through the Digital Store, Xbox One Marketplace, or PlayStation Store, the retail game code will automatically be applied to your account and you should be able to begin the download / launch the game.
If it is, error may have occurred because you have not yet redeemed a retail game code to your account or your access to the game has been restricted. If you are not able to access Fallout 76, first check our Status Portal and Twitter Support page to verify the service is available.