$ sudo do-release-upgrade -d Checking for a new Ubuntu release Upgrades to the development release are only available from the latest supported release. [t]2020-06-05 14:32:45
$ sudo do-release-upgrade -d Checking for a new Ubuntu release Upgrades to the development release are only available from the latest supported release. [t]2020-06-05 14:32:45
英語のリリースノートを見たほうが良さそう。-d オプションは不要だったみたい。
"Launch the upgrade tool with the command sudo do-release-upgrade on 19.10; use sudo do-release-upgrade -d if you are using 18.04 LTS."