However in practice we got an error GPG error: http://deb.opera.com stable InRelease: The following signatures couldn't be verified because the public key is not available: <PUBKEY>
while running sudo apt-get update command.
How to fix:
Here are the steps to solve this problem
1. Execute the following commands in terminal
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY> where <PUBKEY> is the key which will appears with the error on terminal.
2. Now run
sudo apt-get update
This will solve the GPG error: http://deb.opera.com stable InRelease: The following signatures couldn't be verified because the public key is not available: <PUBKEY>
Thanks for the reading !!!