This is technically
story related and can't be missed. Throughout the game you'll be able to upgrade your character's skills. Being lifted paraphrased from GamePressure Guides, there's six skill trees. Four are for you, one is for the squad, and one is for the rebels.
Simply put, unlock a skill for the rebels... and there's an achievement.
For more on skills check this out. https://guides.gamepressure.com/ghost_recon_wildlands/guide....