GPG error: ... The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C718D3B5072E1F5

>PROBLEM

When you run the command:

apt-get update

Returns the following message:

Get:1 http://repo.mysql.com/apt/debian stretch InRelease [21.6 kB]

Ign:1 http://repo.mysql.com/apt/debian stretch InRelease

Fetched 21.6 kB in 0s (87.0 kB/s)

Reading package lists... Done

W: GPG error: http://repo.mysql.com/apt/debian stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C718D3B5072E1F5

W: The repository 'http://repo.mysql.com/apt/debian stretch InRelease' is not signed.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

>SOLUTION

ATTEMPT #1 - UPDATING

apt-key adv --keyserver keyring.debian.org --recv-keys 8C718D3B5072E1F5

Check:
apt-get update

If the issue is not solved, try next alternative.

ATTEMPT #2 - DELETING VIA COMMAND

apt-key del 8C718D3B5072E1F5

Check:
apt-get update

If the issue is not solved, try next alternative.

ATTEMPT #3 - DELETING APT SOURCE FILES

Prerequisite:  supposing that the application is not installed and last removal left garbage behind.

Remove the apt's source files.

>Example:

Get a list of apt's sources:

ls /etc/apt/sources.list.d/*

Returns:

/etc/apt/sources.list.d/mysql.list  /etc/apt/sources.list.d/mysql.list.save

Create a temporary file to enable rollback if the procedure fails and causes malfunction:

mkdir /home/todel

Remove the respective sources causing the issue from apt's folder:

mv /etc/apt/sources.list.d/mysql.list /etc/apt/sources.list.d/mysql.list.save /home/todel

>ENV

debian 9