X package referred to by another package, but cant find other package

I am simply trying to install pdfunite which is inside of poppler-utils; however, I can't seem to figure out how to install poppler-utils. I've tried to install it with apt get install and it just says that it's referred to by another package:

[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

According to another post, I could run apt-cache search xyz to find out what other package it's referred by, but this returns no results:

[root:7e2d33988106:/]# apt-cache search poppler-utils                                                                                                                                                                                                                    
[root:7e2d33988106:/]#

I'm not quite sure what else there is to do. I've tried to run apt get update and re-run apt install poppler-utils, but I get the same results.

[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.

Here's an output of my /etc/apt/sources.list file (I commented out a few lines that had restricted in them:

[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

Here's my ubuntu version:

[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

Are there any other ways I can get poppler-utils installed? I feel like I've run out quite of options here.