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The Advanced Packaging Tool, or APT, is a free user interface that works with core libraries to handle the installation and removal of software on the Debian GNU/Linux computer operating system and its variants.

Missing public key[]

If your system lacks the public key for APT, it will give something similar to the following errors:

W: GPG error: testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: There is no public key available for the following key IDs:

You can fix this by getting the public key with the following commands (remember to replace the key ID with what you need):

sudo gpg --keyserver --recv-keys 9AA38DCD55BE302B
sudo gpg --export --armor 9AA38DCD55BE302B | sudo apt-key add -
sudo apt-get update