Security Unit unlock after certain main mission

There will be an event at mission 21 that will start mission 22 which unlocks the FOB (Forward Operating Base) element of the game.

When this is unlocked, you will get the security team added as this team is the one that defends the base. The security team level is based on the combat skill of the members in the team.

The Security Team becomes available after Mission 22, which introduces the FOB mechanics. It is, in fact, separate from the Combat Team, though it also uses Combat ability as its rank-criteria statistic.

You're almost there!

You're very close - it unlocks after Mission 22. That's all I can say without getting into spoiler territory.