Você deve adicionar a chave pública do repositório "playonlinux":
wget -q "http://deb.playonlinux.com/public.gpg" -O- | sudo apt-key add -
depois disso, execute:
sudo apt update
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://deb.playonlinux.com precise InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 73F0D6E88E3D6C3A
W: Failed to fetch http://deb.playonlinux.com/dists/precise/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 73F0D6E88E3D6C3A
W: Some index files failed to download. They have been ignored, or old ones used instead.
Você deve adicionar a chave pública do repositório "playonlinux":
wget -q "http://deb.playonlinux.com/public.gpg" -O- | sudo apt-key add -
depois disso, execute:
sudo apt update