Seems puppetlabs is trying to force people to the puppet core product, making live difficult for existing puppetlabs users.
Lately the “official” apt GPG key expired at apt.puppetlabs.com, and trying to find help in the puppetlabs forum will give you the idea, that you need to use their repoe, which requires a license.

Until now, the fix for at least Ubuntu, and probably other debian based distroes is to add the DEB-GPG-KEY-FUTURE found here: https://apt.puppetlabs.com/DEB-GPG-KEY-future
Add it with the following command:

wget -O- https://apt.puppetlabs.com/DEB-GPG-KEY-future | sudo apt-key --keyring /etc/apt/trusted.gpg.d/puppet8-keyring.gpg add

You might need to clean up the old key, I overwrote it (same file name), and it worked as expectd.