Security Updates(apt-get updates) or Turnkey(install-security-updates)

run: apt-get updates

returns something like: There is no public key available for the following key IDs: 3B4FE6ACC0B21F32

Fix: sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

Then: apt-get updates

Fixed!(hopefully)