Keeping it simple read along.

EA and Ubisoft have their OWN clients, that they use as DRM for verification, please download the Ubisoft Client directly from Ubisoft.

Now login to your existing Ubisoft account that YOU have previously LINKED your Ubisoft game to, unless this is the first time in HISTORY on your steam account for SAID game, then make sure Ubisoft client is running and login to your Ubisoft account. Launch the game, and it will ask if you want to link it to your Ubisoft account, click accept / yes, then it be on your Ubisoft account, and the game will run.

If for gods knows why you made multiple Ubisoft accounts, and you can't remember which Ubisoft account you have previously LINKED the game to it, you have to figure out by logging into each Ubisoft account until you figure out what one you had used, or contact Ubisoft support for help as only Ubisoft support can help with Ubisoft accounts.

Hope this helps.

You almost certainly need to update or install Ubisoft Connect and get logged in there. It's a requirement for most Ubisoft games, and without it you'll get that activation code prompt.

Might be nice though if ubisoft mentioned "If you're seeing this you should try X or Y", but alas.