Your output of apt-cache policy discord indicates that you have installed Discord using local deb-file. The system is not configured to have special sources.list for Discord, moreover Discord does not provide it.
So if you want automatic updates of Discord you have two options:
install Discord from FlatPak with
sudo apt install flatpak
flatpak remote-add --...
You missed d while installing the package, i.e., tftp-hpa is TFTP client while tftpd-hpa is the server.
If you run apt show tftp-hpa and read description, you'd see
Description: HPA's tftp client
Trivial File Transfer Protocol (TFTP) is a file transfer protocol, mainly to
serve boot images over the network to other machines (PXE).
tftp-hpa is an ...
I checked the original source and it seems you can install caffe with
sudo apt install caffe-cuda
sudo apt install caffe-cpu
As per https://packages.ubuntu.com/, it also gives the same information about both packages:
bionic (18.04LTS) (metapackages): Fast, open framework for Deep Learning (Meta)
1.0.0-6build1: amd64 ...
Why is the candidate different?
Because the priorities of the repositories are different in each system, which result in different candidate packages from different repositories. In the first system the Ubuntu repository has 1000 priority, and the PPA has 500 priority. In the second system both repositories have 500 priority.
If you want to achieve ...
When turning on your computer, you should get a grub screen that has the option to boot in safe mode.(*Here it is Advanced Options for Ubuntu *)
Select that and it would take you to a bare terminal with root privileges. You can then Run Timeshift from terminal