Hello, I'm sorry to hear that you're encountering difficulties launching Rainbow Six Siege. If you are facing an "ownership authentication"[support.ubisoft.com] error, this indicates that the Ubisoft account linked to Steam may not have the necessary game (or DLC) activation. This does not mean that you don't own the game. Fortunately, resolving this is straightforward! If you have a secondary Ubisoft account, simply log in to that account and relaunch the game.

If you're uncertain about the existence of an alternate account (possibly due to a misspelled email address), please reach out to the Ubisoft Support team on the Help Site[www.ubisoft.com]. Provide them with your launcher logs[support.ubisoft.com], an uncropped screenshot of your Steam Proof of Purchase[www.ubisoft.com], and your Steam ID[support.ubisoft.com]. They will assist you further in resolving the issue!