我只是想安装pdfunite其中的poppler-utils;但是，我似乎不知道如何安装poppler-utils。
我尝试安装它，apt get install但它只是说它被另一个包引用：
[root:7e2d33988106:/]# apt install poppler-utils -y 4:30AM/09.12
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package poppler-utils is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'poppler-utils' has no installation candidate
根据另一篇文章，我可以运行apt-cache search xyz找出它所引用的其他包，但这不会返回任何结果：
[root:7e2d33988106:/]# apt-cache search poppler-utils
[root:7e2d33988106:/]#
我不太确定还有什么可做的。我尝试运行apt get update并重新运行apt install poppler-utils，但得到相同的结果。
[root:7e2d33988106:/]# apt update 4:35AM/09.12
Hit:1 https://deb.nodesource.com/node_6.x bionic InRelease
Hit:3 http://archive.ubuntu.com/ubuntu bionic InRelease
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:4 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Reading package lists... Done
E: Release file for http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease is not valid yet (invalid for another 16h 57min 47s). Updates for this repository will not be applied.
E: Release file for http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease is not valid yet (invalid for another 16h 58min 26s). Updates for this repository will not be applied.
这是我的文件的输出/etc/apt/sources.list（我注释掉了其中的几行restricted：
[root:7e2d33988106:/]# cat /etc/apt/sources.list | grep -v "#" | sort -u 4:36AM/09.12
deb http://archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://archive.ubuntu.com/ubuntu/ bionic universe
deb http://archive.ubuntu.com/ubuntu/ bionic-updates multiverse
deb http://archive.ubuntu.com/ubuntu/ bionic-updates universe
deb http://security.ubuntu.com/ubuntu/ bionic-security multiverse
deb http://security.ubuntu.com/ubuntu/ bionic-security universe
这是我的ubuntu版本：
[root:7e2d33988106:/]# lsb_release -a 4:36AM/09.12
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.2 LTS
Release: 18.04
Codename: bionic
还有其他方法可以poppler-utils安装吗？我觉得我已经没有太多选择了。