Ubuntu Err :18 http://dl.google.com/linux/chrome/deb stable Release.gpg KEYEXPIRED 1555048520

This is an ongoing problem that only seems to hit Google Chrome. In order to avoid seeing warnings, one can temporarily turn off the repository for GC.

To get rid of the problem (temporary) use --allow-unauthenticated in the apt command and wait until google fix the signing key.

You can check it from time to time by calling

curl 'https://dl.google.com/linux/linux_signing_key.pub' | gpg

wget -q -O - https://dl.google.com/linux/linux_signing_key.pub | sudo apt-key add -

https://www.google.com/linuxrepositories/