コマンドを実行して python3 を削除しようとしましたsudo apt-get remove python3が、システム全体が壊れてしまいました。再インストールなど、インターネットで見つかった複数の解決策を試しましたubuntu-descktop。
sudo apt install --reinstall ubuntu-desktop
結果：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
ubuntu-desktop : Depends: gdm3 but it is not going to be installed
Depends: gnome-control-center but it is not going to be installed
Depends: gnome-shell but it is not going to be installed
Depends: gnome-shell-extension-appindicator but it is not going to be installed
Depends: gnome-shell-extension-desktop-icons but it is not going to be installed
Depends: gnome-shell-extension-ubuntu-dock but it is not going to be installed
Depends: ubuntu-desktop-minimal but it is not going to be installed
Depends: ubuntu-session but it is not going to be installed
Depends: yelp but it is not going to be installed
Recommends: gnome-getting-started-docs but it is not going to be installed
Recommends: ibus
Recommends: ibus-table but it is not going to be installed
Recommends: plymouth-theme-spinner but it is not going to be installed
Recommends: ubuntu-docs but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
依存関係を手動で再インストールすることも試みましたが、うまくいきませんでした。