Ran into these errors while trying to install Sphinx on 18.04:
On running sudo apt-get update got the following error message
GPG error: http://plf.parrot.com/sphinx/binary bionic/ InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 47727EDC508B1AE5
E: The repository ‘http://plf.parrot.com/sphinx/binary bionic/ InRelease’ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
What should I do?