Was having this issue earlier. Tried several times to redownload it, but no change. Just a couple minutes ago, the patch installed itself and finished randomly, and I'm able to play the game. Could be server stuff on their end connecting to us. I say just give it a little bit of time and see if it works. I also restarted Uplay a few times as well.
Hello Agents,
Sorry to hear that you are unable to download the update.
Can you please try all the troubleshooting steps listed here? Many thanks and apologies for any inconvenience caused by this.
If you have any additional questions, please let us know.