Hey @JTdawg12, and welcome to the Discussions.
The ownership of a game is bound to the Ubisoft account. Meaning that if you launch Rainbow Six : Siege through Steam while not being connected to the account it was activated on, the system won't be able to authenticate ownership.
If you wish to play on another account, that said account must own the game.
However, if you require a game transfer, you can always ...