A few users have reported they get 'UBISOFT REDEMPTION ERROR' when they try and activate. They then go on to report that after repeatated attempts the game activates correctly.
We have a suppport ticket open with UBI at the moment so they can fix their connection for us.
Please can you try logging into Fanatical using a different browser and attempting your activation again. Some users report that this fixes the problem, but the general rule is to simply re-try every few minutes.