I'm using Tails, which I think is Debian Stretch, right? I can reach websites on my browser, but I can't ping anything, not even loopback.
I'm trying to get I2P up and running. I get this error when I do a 'sudo apt-get update' -

Err:1 https://deb.i2p2.de stretch InRelease
Could not resolve host: deb.i2p2.de

#Then the updates that DO work, then,

W: Failed to fetch https://deb.i2p2.de/dists/stretch/InRelease Could not resolve host: deb.i2p2.de

I imported the key, like their instructions said on https://geti2p.net/en/download/debian#debian.

I can even open the output from https://deb.i2p2.de/dists/stretch/InRelease..... it begins like this:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Origin: I2P
Label: I2P
Suite: newstable
Codename: stretch
Date: Wed, 27 Jun 2018 17:14:53 UTC
Architectures: i386 amd64 armel armhf
Components: main
Description: I2P
MD5Sum:
7fd03633dcb03ecfe598ec84bb4d9be4 5049 main/binary-i386/Packages
3831d875db3b16111bb8ab001633fb2f 1936 main/binary-i386/Packages.gz

Is this maybe a PGP issue?
And why can't I ping?
Sorry for the long post.

I am able to ping this domain.
Are you sure you are not behind any proxy or your terminal is correctly configured for a proxy?
Are you sure apt is correctly configured or it is not corrupted because of your unnecessary changes?

Your firewall is probably blocking ping. Are you able to ping any other site, like google.com? How did you add I2P repo to sources.list? Manually? Try changing it from https to http. And look in 'iptables -L' to see if port 443 is blocked.

@OP
This message means that your dns resolver cannot resolve the hostname deb.i2p2.de.
I'm not familiar with Tails, but this seems to be relevant to your problem.

Regards