This is the error I get after I launch from steam
https://imgur.com/UsAs3Bt
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What Gabbin is mentioning is that it's likely you first played the game on a different Uplay account. So you'll have launched it on Steam, and then logged into a Uplay account as this is required to play.
For assistance with this I'd advise speaking with the Support Team through the site here, as they should be able to help you locate the account.
Thank you