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If below error happened while execute do-release-upgrade.
do-release-upgrade
Checking for a new Ubuntu release
Your Ubuntu release is not supported anymore.
Follow below step to fix.
Run below command
sudo apt upgrade
And then
sudo apt dist-upgrade
Then try again do-release-upgrade.
Refer – https://askubuntu.com/questions/1390585/my-ubuntu-says-i-should-upgrade-but-then-doesnt-let-me