@rakemup,

It looks like Mint can’t find those packages because they’re not in the default repos.

A couple of things you can try:

1. Update your package list first:

sudo apt update

2. Fastfetch, is a newer tool, so better install it from the official PPA or GitHub release:

sudo add-apt-repository ppa:zhangsongcui3371/fastfetch
sudo apt update
sudo apt install fastfetch

3. Similary for foliate, you need to install it from its own PPA:

sudo add-apt-repository ppa:apandada1/foliate
sudo apt update
sudo apt install foliate

Alternatively, both apps are on Flatpak, which Mint supports out of the box:

flatpak install flathub com.github.tchx84.Fastfetch
flatpak install flathub com.github.johnfactotum.Foliate

That should get them working on Mint XFCE

$ sudo apt install fastfetch
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package fastfetch

And:

$ sudo apt install foliate
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package foliate

What am I doing wrong?
Thanks!