Hey @EricFromFasty, thank you for posting.
The authentication error is working as intended, and is therefore not a defect.
The error appears when the ownership of the product cannot be authentified.
If you are not logged into the Ubisoft account where the game has been bought from, you won't be able to launch the game.
Since the game is available on multiple platform, the signed-in accounts must match the accounts the game has been bought with.
If this error persists, it means that you are not connected to the rights accounts. Can be the Steam one, or the Ubisoft one. To get help regarding this issue, please contact us directly so we can have a look through your account and investigate further for yourself...