Невозможно обновить сервер с 18.04.05 до 20.04.01 [дубликат]

17:37UTC:root@bard:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.5 LTS
Release:        18.04
Codename:       bionic

17:38UTC:root@bard:~# do-release-upgrade 
Checking for a new Ubuntu release
There is no development version of an LTS available.
To upgrade to the latest non-LTS develoment release 
set Prompt=normal in /etc/update-manager/release-upgrades.`

Почему мой менеджер обновлений сообщает, что "разрабатываемая" версия LTS недоступна?

17:41UTC:root@bard:~# cat /etc/update-manager/release-upgrades
# Default behavior for the release upgrader.

[DEFAULT]
# Default prompting behavior, valid options:
#    
#  never  - Never check for, or allow upgrading to, a new release.
#  normal - Check to see if a new release is available.  If more than one new
#           release is found, the release upgrader will attempt to upgrade to
#           the supported release that immediately succeeds the
#           currently-running release.
#  lts    - Check to see if a new LTS release is available.  The upgrader
#           will attempt to upgrade to the first LTS release available after
#           the currently-running one.  Note that if this option is used and
#           the currently-running release is not itself an LTS release the
#           upgrader will assume prompt was meant to be normal.
Prompt=lts

В этой системе есть [как мне кажется] общий /etc/apt/sources.list файл. Указывает на различные ubuntu.com Bionic-репозитории.

??