There are three reasons why Steam may be asking you for a Ubisoft Connect key.
Here we provide some steps on how to fix this issue -

• You may be running an outdated version of Ubisoft Connect. The new version can be downloaded here -
  https://ubisoftconnect.com/
  
  
• You have not activated your Steam game on Ubisoft Connect yet.
  To learn how to do this, please check out this link -
  https://support.ubisoft.com/en-GB/Article/000060587#uplay
  
  
• You may need to delete your Ubisoft Connect Cache - https://ubi.li/eGZul
  Next, verify your game in Steam by following this guide - https://ubi.li/qJNkE
  Now try relaunching the game to check if it asks you for a CD key?
  
  
• You are logged into a different Ubisoft account than the one the game was originally activated on. You may see a "There was a problem authenticating the ownership of this product" message. Try logging out of both game launchers then logging back in.

If you are affected by the 4th option, this usually means the game is connected to either a different Steam account or a different Ubisoft account, if you have more than just one of each.

In order to help you find the correct one you would need to get in touch with us through our support channels, where we can help you locate the correct Ubisoft account that is tied to the game activation.

When you contact us please let us know the names of your Ubisoft accounts, any emails you have previously used ,any Steam ID's which you have used with the game, what game you are unable to access and any other platforms such as console you may have linked.

You can contact us here through private message over our social media or our Ubisoft Support website -

Twitter Support - https://twitter.com/UbisoftSupport
Facebook Support - https://www.facebook.com/UbisoftSupportUK/
Ubisoft Support Website - https://support.ubisoft.com/en-gb/Cases/New