What types of enemies can’t take cover

There are 5 types of Experimental Ammo, and 3 types of Experimental grenades.

Ammo

As a side note, Ammo applies to Sharpshooter pistol shots.

• Armor Piercing - Bypasses an enemy's armor. No bonuses against enemies without armor.
• Dragon Rounds - Passive bonus damage and a chance to light targets on fire. (Enemies immune to fire can't be lit on fire, but still take the bonus damage)
• Talon Rounds - Reaper Rounds' big brother! +20% passive crit, and an extra +1 damage on critical hits. No extra range penalties like its predecessor.
• Tracer Rounds - +10 Aim while equipped.
• Venom Rounds - Passive bonus damage and a chance to poison the target. (Enemies immune to poison, like Chyrssalids, still take the bonus damage)

Grenades

• Acid Grenade / Bomb - Shreds more armor than a frag grenade, applies Acid to those hit, and leaves acid terrain tiles on the map for a few turns.
• Gas Grenade / Bomb - Larger area grenade that deals damage and poisons enemies hit.
• Incendiary Grenade / Bomb - High damage and sets enemies on fire.

And, though you didn't ask for it, why not --

Heavy Weapons

Heavy weapons are tiered. The "Heavy Weapons" become available once you unlock the E.X.O. suit, and the "Powered Weapons" become available once you unlok the W.A.R. suit.

Heavy Weapons

• Rocket Launcher - Unlocked by default. Deals explosion damage in an area & shreds armor.
• Flamethrower - Cone of flame to deal damage and ignite enemies.
• Shredder Gun - Cone of bullets to shred armor and cover.
• Blaster Launcher - Powered Weapon. Upgraded Rocket Launcher.
• Hellfire Projector - Powered Weapon. Upgraded Flamethrower.
• Shredstorm Cannon - Powered Weapon. Upgraded Shredder Gun.
• Plasma Blaster - Powerd Weapon. Linear area damage.

Here's an imgur album assembled by SYLOH with the in-game descriptions.

Since no one (other than Firaxis) can know for sure, here's my interpretation of your picture:

The soldier you're trying to target isn't standing in the open door, but next to it. He's standing in the same line as your soldier, and the full-cover-box you can see behind the doorway.

I think it's that specific box that is blocking your LOS. Even if we consider that your soldier sidesteps (so let's assume she's standing one tile to her left); it's possible that the slight elevation of the train cart combined with the angle of the box (relative to how your unit would see the enemy soldier) leads the game to decide it's not in LOS.

That being said, this is just a guess. I remember being in similar situation and seeing my target. The only thing different here is that box, which is why I'm thinking that's the reason.

Edit
For those who argue that the enemy (model) is in sight of the soldier, therefore it should be targetable, keep in mind that the game calculates based on the tile a unit is on, not the actual visibility of their character model.

I find that units in cover really cling to the cover, or try to pop out their heads, and it throws off your vision on where the tile is located. If you look at OP's picture, you'll see how much difference there is between the enemy's body and the actual center of the tile they're standing on.