W: GPG error: http://apt.llvm.org/eoan llvm-toolchain-eoan InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 15CF4D18AF4F7421
E: The repository 'http://apt.llvm.org/eoan llvm-toolchain-eoan 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.