Ubuntu 17.04 is very close if not already EOL. But I was just tidying up my VMs and ran into this:
Don't the repositories hang on for awhile? I recall a year or so. I've been seeing this for weeks.
The support for 17.04 has stopped since january 13. If you check http://us.archive.ubuntu.com/ubuntu/dists/ you’ll see that there are indeed no more repos for zesty, thus the message you have.
I’m not sure about past policies but I guess that Canonical removes the obsolete repos immediately after support ends.
If you need them, they’re still available on http://old-releases.ubuntu.com/ and you can update your sources list accordingly. You should definitely upgrade to 17.10, though.
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Thanks, the old-releases is new to me and I’ve been playing with Ubuntu since Dapper (which is there!). 
I know 10.04 repos stayed for a loooong time after EOL but that’s an LTS.
I keep old VMs around for reference, it’s not in active use. I think this repo issue goes back before Jan 13, though.