that is the big anniversary update: all my PCs bar one installed it ok, but 1 just stubbornly refused & always errored out, then rolled back- and I tried every possible workaround / repair.
after about a week of fighting that ,I took out the C drive, did a new clean install to a new SSD instead... job done
you may have a similar case, but one workaround is to use the media creator tool from Microsoft & download win 10 AU to DVD or flash drive , rhen update ftom that rather than via windows update - didn't work for may problem PC but it might just work for yours..
or you can just ignore it, you wont' get AU or any future updates but if you are OK with that... & there's a trick to make it stop trying which I used and posted a while back. you make a registry change that cases MS to think your wired connection is metered, then it stops trying to auto-install. if you are on wifi its easier just tick I have a metered connection in settings.