If you find that the game is shown in your Uplay account as "Installed" but cannot be launched and appears dimmed or greyed out - this indicates the game is installed on your computer but not owned by the Uplay account you're logged into.
So there will be another Uplay account on this PC and you need to sign out of the correct Uplay account then sign into the Uplay account that does own the game so you can launch and play it.