Why does ubuntu download verification require 2 keys, and why do I only have 1?
Solution 1:
The error message indicates that you do not currently have the Ubuntu CD signing public keys in your GPG Keyring. The instructions on the page you linked-to include steps for importing the key in such situations:
gpg --keyid-format long --keyserver hkp://keyserver.ubuntu.com --recv-keys 0x46181433FBB75451 0xD94AA3F0EFE21092
This will download the key and add it into your GPG Keyring ready for use. From there you will be able to re-run the first command again, and it should succeed:
gpg --keyid-format long --verify SHA256SUMS.gpg SHA256SUMS