每次我输入这一行：
sudo pkexec synaptic
突触包管理器打开，但始终伴随以下错误消息：
E: Could not open file - open (2: No such file or directory)
E: _cache->open() failed, please report.
如果我关闭错误框，突触将同时关闭。我该如何修复这个错误并最终启动 synaptic？
@Henrik，我刚刚跑了
sudo apt update
这是在终端中生成的完整响应：
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.