Problem:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cdn-fastly.deb.debian.org/debian stretch-b
ackports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
W: Failed to fetch http://http.debian.net/debian/dists/stretch-backports/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
W: Some index files failed to download. They have been ignored, or old ones used instead.
Solution:

apt-get install debian-keyring debian-archive-keyring