[2020] I bought R6 3 years ago in steam and now it wont let me link my uplay acc

Hey there, shacklebot24! I'm sorry to hear that you haven't been able to play Rainbow Six Siege due to the authentication error you're receiving. Since the game has already been activated under a separate account, you wouldn't be able to play it from a separate account. If you don't remember the details for the account that the game was initially activated under, please navigate to the following directory on your PC:

C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\cache\ownership

Then, grab a screenshot of that folder's contents and upload it in a case at support.ubi.com. You can also contact our team on Facebook or Twitter. From there, they will be able to use this screenshot to investigate this further and ultimately provide the information for the account with Siege. Please let me know how you decide to reach out to our team. Thank you!