Unfortunately, when you’re first starting out in Deus Ex: Mankind Divided, you won’t be able to perform melee attacks. There just simply isn’t an option to do that. Just like in the first game, the only way to perform a melee attack is by getting up close and personal with an enemy and tapping the ‘Takedown’ button. This will allow Jensen to take out the enemy, either lethally or non-lethally.
On PC, you’ll want to tap the Q button once you’re near an enemy to perform a takedown. However, do take note that you can’t just perform takedown after takedown. After using it once, there will be a short cooldown timer that has to count down before you can use that ability again, so make sure to use it wisely. You’ll be able to perform takedowns from cover once an enemy is near your position, and you can also do it even when the enemies have been alerted to your presence. I’ve found that even after enemies detect you, you’ll still be able to run up to them and quickly tap the button to take them down before they can even fire at you.
This ability is useful both in and out of stealth, so make sure to use it often instead of just relying on your stun gun in Deus Ex: Mankind Divided.