Contact the place where you purchased it. Steam do not sell game keys, so this has nothing to do with Steam

Install Ubisoft Connect. Create a Ubisoft account. Log in. Link your Steam account. Launch the game from Steam.

yeah look for where is the key maybe is for ubisoft and is not for steam, or maybe you have the demo version and to play more you need the key to activate all when or where it ask you to put the code?

Modern Ubisoft games do not use cd-keys. They are activated via the Uplay API system which links a Ubisoft account to Steam the process is

1) Launch game on steam
2) Steam sends an API call to Ubisoft Connect
3) Ubisoft connect launches with an account you created
4) Ubisoft connect registers the game on that Ubisoft Connect account
5) Records that your Steam account owns that game on that Ubisoft account

When you launch the game in the future

1) Launch game on steam
2) Steam sends API call to Ubisoft Connect
3) Ubisoft connect checks that
A) That Ubisoft account owns that game
B) That game is registered to that particular Steam account

Cd-keys are no longer needed for Ubisoft games purchased on Steam