Certificate validation error when updating an expired GPG key in Puppet Enterprise 2016.4.0 to 2019.8.4

My Puppet Enterprise GPG key expired. I am trying to update it, but when I run apt-get update to update the package list, I get a certificate verification error.

Error message

E: Failed to fetch https://puppet.example.com:8140/packages/2018.1.18/ubuntu-18.04-amd64/dists/bionic/puppet5/binary-amd64/Packages  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 10.131.15.99 8140]
E: Some index files failed to download. They have been ignored, or old ones used instead.
E: Conflicting values set for option Trusted regarding source https://puppet.example.com:8140/packages/2018.1.18/ubuntu-18.04-amd64/ bionic
E: The list of sources could not be read.
+ DEBIAN_FRONTEND=noninteractive
+ apt-get install -y -o Apt::Get::Purge=false -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-confdef --no-install-recommends 'puppet-agent=5.5.22*'
E: Conflicting values set for option Trusted regarding source https://puppet.example.com:8140/packages/2018.1.18/ubuntu-18.04-amd64/ bionic
E: The list of sources could not be read.
E: Conflicting values set for option Trusted regarding source https://puppet.example.com:8140/packages/2018.1.18/ubuntu-18.04-amd64/ bionic
E: The list of sources could not be read.

You get this error because there is a certificate trust conflict for the source URL of the file.

Version and installation information

PE version: 2016.4.0 to 2019.8.4
OS: Debian, Ubuntu, SLES 15

Solution

Based on the error, on the primary server, in your source file list (/etc/apt/sources.list), check sources.list for conflicting lines, resolving any issues that you find for Puppet repositories. If you find duplicates, remove or comment them out. Make sure that the source you want is trusted, for example:

deb [trusted=yes] https://puppet.example.com:8140/packages/2018.1.18/ubuntu-18.04-amd64 bionic puppet5