For Flakk missiles: Armies have the upgrade options listed in their codex and FAQ/Errate document. If the Flakk missile upgrade does not appear in either of those places, your army cannot currently take that upgrade.
This can't be accurate, as no current codex or the recent faq/errata mentions anything about Flakk missiles. They are only mentioned in the main rulebook
as a sub munition for the
Missile Launcher. To my knowledge; Space Marines, Chaos Marines, and Imperial Guard are the only armies with access to that weapon.
The rule book contradicts itself, it says it an upgrade but the profile for the Missile Launcher was actually changed to
include Flakk rounds.