この行を入力するたびに：
sudo pkexec synaptic
シナプティックパッケージマネージャが開きますが、常に次のエラーメッセージが表示されます。
E: Could not open file - open (2: No such file or directory)
E: _cache->open() failed, please report.
エラーボックスを閉じると、シナプスも閉じます。このエラーを修正して最終的にシナプティックを開始するにはどうすればよいですか？
@Henrik、ちょうど走った
sudo apt update
端末で生成された全体的な応答は次のとおりです。
Ign:1 http://ftp.us.debian.org/debian stretch InRelease
Get:2 http://deb.debian.org/debian stretch-backports InRelease [91.8 kB]
Hit:3 http://ftp.us.debian.org/debian stretch Release
Get:5 http://deb.debian.org/debian stretch-backports/main amd64
Packages.diff/Index [27.8 kB]
Get:6 http://deb.debian.org/debian stretch-backports/main amd64 Packages
2018-12-24-0808.38.pdiff [622 B]
Get:6 http://deb.debian.org/debian stretch-backports/main amd64 Packages
2018-12-24-0808.38.pdiff [622 B]
Fetched 120 kB in 1s (75.7 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
2 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: http://ftp.us.debian.org/debian/dists/stretch/Release.gpg: The key(s)
in the keyring /etc/apt/trusted.gpg are ignored as the file is not
readable by user '_apt' executing apt-key.
W: http://deb.debian.org/debian/dists/stretch-backports/InRelease: The
key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not
readable by user '_apt' executing apt-key.