It looks like the issues that were happening on GeForce Now with Assassins Creed Origins and Assassins Creed Syndicate are now resolved. There have been a number of issues with the games over the last number of months including the games freezing or not loading. The issues with AC: Origins appear to be mostly gone, (I was getting some frame drops). For AC: Syndicate there does seem to be a little issue while loading into the game, it says connection error, but after this goes the game appears to run fine.
This fix was likely due to a graphics card update. While this not be populated to all the GeForce Now servers hopefully it will soon and you will be able to play both Assassins Creed games without any issue.