Upgrading guppy basecaller on Oxford Nanopore Gridion via PPA
This is an ONT problem, not an ubuntu problem. Keys can expire over time, and need to be updated.
The keys for the Guppy basecaller are not present in the ubuntu repository; you need to get them from ONT. This is indicated by the installation instructions for Guppy (which you've already linked to):
sudo apt-get update
sudo apt-get install wget lsb-release
export PLATFORM=$(lsb_release -cs)
###
wget -O- https://mirror.oxfordnanoportal.com/apt/ont-repo.pub | sudo apt-key add -
###
echo "deb http://mirror.oxfordnanoportal.com/apt ${PLATFORM}-stable non-free&quot; | sudo tee /etc/apt/sources.list.d/nanoporetech.sources.list
sudo apt-get update