Having the same issue on all my laptops and desktop machines, which use either Windows XP or Window 7; none of them will update!
A similar definitions update problem popped up early last week on two of my machines running Avast, but I was able to resolve it by simply uninstalling and reinstalling Avast 7. A look at Avast's website (» mentions support dropped as of January 1, 2017 for version 4.8.0 has VPS, but there is mentioned about version 5 to 8.
Avast is still running, but its virus definitions are stuck at '170216-0' (Feb. If I try selecting a manual update, it reports "Already up to date (current version 170216-0)".
I next tried downloading the Avast VPS exe from the official site, and get the same type of message.
When this happens you will also get a pop-up notification saying ‘But sometimes you need to update virus definitions on a computer which has no Internet access at all.