If you are experiencing errors when attempting to download your game through the Uplay client, 

First, Try running Uplay as administrator, you can do this by right clicking your Uplay icon on the desktop, and selecting Run as Administrator

If this does not help, you may wish to check your firewall settings to make sure that your Uplay client is not being blocked by any protective software. You can find a list of ways to configure popular firewall software HERE.