It fails with missing public key.
08:42:44 Err:6 https://downloads.apache.org/cassandra/debian 40x InRelease 08:42:44 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 50EE103D162C5A55 08:42:44 Get:8 http://incoming.debian.org/debian-buildd buildd-unstable/main amd64 Packages [199 kB] 08:42:45 Reading package lists... 08:42:45 W: GPG error: https://downloads.apache.org/cassandra/debian 40x InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 50EE103D162C5A55 08:42:45 E: The repository 'http://www.apache.org/dist/cassandra/debian 40x InRelease' is not signed. 08:42:45 E: apt-get update failed