Hello everyone!
From your descriptions it seems as though it is the linking stage we are having issues with. If we can get round that, you should be able to play.
I have a trick that will hopefully help for you all -
1. Go to the Ubisoft Club website:
https://club.ubisoft.com/ and make sure you're signed out
2. Click to Sign in
3. Select the Xbox icon and sign in with your Xbox account details
If your Xbox account is already linked to a Ubisoft account, it will sign you in without issues. If not, it will ask you which Ubisoft account you would like to link it to.
I hope that helps but let me know ...
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