Hello LuciaXV,
Is your Steam account already linked to a different Uplay account? There's no codes anymore between Steam and Uplay it will automatically add it to your account when you launch the game and login to Uplay.
That being said if you Steam is already linked to a Uplay account, it may encounter problems if you try to use a different Uplay account.
Thank you
Thank for for confirming this.
In that case, please speak with the Support Team through one of the following channels:
They will be able to look into this and provide you with assistance.
Thank you