The repository 'http://repo.mysql.com/apt/ubuntu bionic InRelease' is not signed.
Err:8 http://repo.mysql.com/apt/ubuntu cosmic InRelease
The following signatures were invalid: EXPKEYSIG 8C718D3B5072E1F5 MySQL Release Engineering <[email protected]>
SOLUTION
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8C718D3B5072E1F5