This question already has an answer here:
I am currently using Ubuntu 15.04 and wish to upgrade to 16.04 LTS. I try:
But I get:
Checking for a new Ubuntu release No new release found
How do I move forward? Is my install simply reporting the wrong version number - how can I find out?
I mean, it looks like I am still on 15.04
lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 15.04 Release: 15.04 Codename: vivid
What next steps can I take?
I followed the directions in How to install software or upgrade from an old unsupported release? which caused a lot of 404 errors. I then realised that vivid was not in http://old-releases.ubuntu.com and search replaced those urls back to http://gb.archive.ubuntu.com/ and, bingo, updates working as they should.
TL;DR: The sources list was clearly wrong. Fixed it.