Yes, that's happening on my system as well. In fact, I did a direct download of the IPSW files, and iTunes would not download them. When I clicked "Update", I got the same pop-up. It also quit iTunes, which I thought was weird.
P.S. Mine is holding a 1 minute, 17 seconds remaining.
[doublepost=1537228716][/doublepost]It looks like this is a forced iTunes update. I now seem to have iTunes 12.8.0 installed now.