Fail to sudo apt-get update [duplicate]
You should use the --allow-unauthenticated option to skip it:
sudo apt-get --allow-unauthenticated update
sudo apt-get --allow-unauthenticated upgrade
Or if you have have the id of the missing key (from the output of apt-get), you can add it to the apt key manager to be authenticate :
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys put_the missing_key_here