When I run :
sudo do-release-upgrade
I get a message saying:
Please install all available updates for your release before upgrading.
I have already run :
sudo apt-get update
The final message here is :
Some index files failed to download. they have been ignored, or old ones used instead.
I then ran:
sudo apt-get upgrade
Towards the end of this output is the following:
The following packages have been kept back: linux-generic linux-headers-generic linux-image-generic 0 to upgrade, 0 to newly install, 0 to remove and 3 not to upgrade.
After this is when I ran :
sudo do-release-update
and that's when I get the message :
Checking for a new Ubuntu release Please install all available updates for your release before upgrading.
I saw this : Can't upgrade Ubuntu 16.04.5 LTS to 18.04.1 LTS because can't upgrade libbluray-bdj
which seemed similar but unlike there the package was inconsequential here the packages seem central to the system.
Any help would be gratefully received.
sudo apt update
indicates a problem ("Some index files failed to download."). Don't move forward until you have solved that.