server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none

Question

I am trying to follow steps described on Tor Project Debian Repository page to install latest LTS tor packages.

When I run the apt update command, I am getting following error

<omitted unrelated lines>
Err:10 https://deb.torproject.org/torproject.org xenial Release
  server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
<omitted unrelated lines>
Reading package lists... Done
E: The repository 'https://deb.torproject.org/torproject.org xenial Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

My Ubuntu version is:

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.4 LTS
Release:        16.04
Codename:       xenial

Answer 1

Xenial packages are no longer available at that repository because Xenial is EOL. Please upgrade to a newer, supported version of Ubuntu (e.g., 18.04, 20.04, etc).

You can either install a supported version of Ubuntu from a new download or, if you're feeling adventurous, you can follow the instructions in this answer to upgrade manually from the terminal.