any way to fix this error ? Im just trying to download the signal app
W: GPG error: https://updates.signal.org/desktop/apt xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D980A17457F6FB06
E: The repository 'https://updates.signal.org/desktop/apt xenial 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.