Hi there i_R_HLG, welcome to the Siege forums!
When you see the message - "There was a problem authenticating the ownership of this product."
This usually means the game is connected to either the wrong Steam account or the wrong Ubisoft account.
In order to help you find the correct one, you would need to get in touch with us through our support channels as we are unable to ask you personal identifiable information over the forums.
Please gather as much of the following information as possible as this will make it easier to locate your account -
Ubisoft email address or previous emails you may have used
Game you are trying to access
Any linked accounts such as PSN / Gamertags / Steam ID code / Epic ID code
We will do our best to locate your correct account where you activated the game.
You can contact us through our support website or through private messaging, via the following links -
Ubisoft Support Website - https://support.ubisoft.com/en-gb/Cases/New
Twitter Support - https://twitter.com/UbisoftSupport
Facebook Support - https://www.facebook.com/UbisoftSupportUK/