2. You are logged into a different Ubisoft account than the one the game is activated on.
3. You are running an outdated version of Uplay PC.
If you haven't activated your game yet, please activate it in Uplay PC. To find out how to find the activation key that you can enter in Uplay PC, please take a look at this official Steam FAQ.
If you activated the game previously and get an error message trying to activate it again, it is likely that you are logged into the wrong Ubisoft account. Please make sure you are logged into the same account that you activated the game on previously. If you are unsure which account the game is activated on, we might be able to help. Feel free to contact us!
However, in many cases it is no longer necessary to activate a Steam game in Uplay PC. Steam changed the way the activation works so you no longer need a key for most games.
To find out if your game does not require a Steam key anymore, please check if Steam provides a key for this game. To find out how to do that, please take a look at this official Steam FAQ. If you cannot find a key for your Steam game in your Steam library, it is most likely not needed. Games without key are automatically redeemed on your Ubisoft account when you run them for the first time. If it does not work right away, you may need to update or reinstall Uplay.
If you have any questions, feel free to contact us!