I had this:
root@big:~# apt-get update […] Get:13 https://packagecloud.io/grafana/stable/debian jessie InRelease [23.4 kB] Err:13 https://packagecloud.io/grafana/stable/debian jessie InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40F370A1F9081B64 Reading package lists... Done W: GPG error: https://packagecloud.io/grafana/stable/debian jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40F370A1F9081B64 E: The repository 'https://packagecloud.io/grafana/stable/debian jessie 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. root@big:~#
This helps:
root@big:~# cat /etc/apt/sources.list.d/grafana.list # deb https://packagecloud.io/grafana/stable/debian/ jessie main deb [arch=amd64] https://packages.grafana.com/oss/deb stable main root@big:~#