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This was after apt update, apt-dist-upgrade:

root@_____:~# cat /etc/os-release 
NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial

root@_____:~# do-release-upgrade 
Checking for a new Ubuntu release
No new release found.
Karel Adams
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  • Did you try to update from 16.04? Is this output after the update? What is the output of cat /etc/apt/sources.list? And what about cat /etc/update-manager/release-upgrades? – Kulfy Jan 05 '20 at 09:47
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    @Kufly: sources.list shows nothing unexpected, all lines (except comments, of course) begin with "deb http:" and all contain "xenial". As for the update-manager file, it only has one single line, apart from the comments: "Prompt=never". I compared with another Ubuntu, already at 18.04, and found no difference. – Karel Adams Jan 05 '20 at 09:56
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    Your Ubuntu would never get updated to newer version of Ubuntu unless you change prompt to lts (to get upgrades to LTS releases only) or normal (to get any upgrade to any release including non-LTS releases). – Kulfy Jan 05 '20 at 10:07
  • Yes, changing the "Prompt=never" to "Prompt=lts" in etc/update-manager/release-upgrades did the trick; upgrade has been started now. Put this as an "answer" and I'll be glad to endorse it. – Karel Adams Jan 05 '20 at 10:17

1 Answers1

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Step 1: Upgrade the packages to the latest version for 16.04 decreases the chance of something going wrong during the upgrade to version 18.04. Run the command below 

sudo apt update && sudo apt upgrade

Step 2: Install the Ubuntu Update Manager.

sudo apt install update-manager-core

Step 3: Upgrade Ubuntu

sudo do-release-upgrade

Step 4: After the upgrade is complete, a restart will be required.

Step 5:To check what version of Ubuntu is currently installed, use the command below.

lsb_release -a

If that does not work, you need to follow these steps

Step 1: Edit apt configuration

Press Ctrl Alt T to open terminal Now, type 

sudo nano /etc/apt/apt.conf

Step 2: Change the config:

APT::Default-Release *;

Note If the apt.conf file does not exist, create your own user file in /etc/apt/apt.conf.d so you can guarantee that it won't be overwritten by package updates. Use the command to create the file

sudo touch /etc/apt/apt.conf.d/apt.conf

Then to edit your file run

sudo nano /etc/apt/apt.conf.d/apt.conf

Add this line to the file

 APT::Default-Release *;
Tejas Lotlikar
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    Needlees to say that the standard procedure had already been tried, also the update-manger-core was already in place so it did not get any upgrade. The alternate procedure did not work, either. /etc/apt/apt.conf did not exist, so I created one in /etc/apt/apt.conf.d, with the suggested content. Renewed apt update && apt upgrade did install libappstream4, so the whole apt chain is working ok. Still, do-release-upgrade comes back with the same "No new release found." – Karel Adams Jan 05 '20 at 09:52
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    @KarelAdams, might be but I do not see that in the question. – vanadium Jan 05 '20 at 11:29