like in games like gears of war, halo,etc. How to set a door to not being able to open until lets say you kill all the enemies. Is a blueprint or coding?
You can do it in your blueprints -> just use a branch node and a bool variable 
Hey do you have any tutorial? So i can check it?
Or here is a simple example
When the player touches the trigger from the door it checks if the integer (amount of keys) is 2 -> when the player has all keys (2) it will open the door. In your key bp you just have to add a capsule + a overlap event which will set the int to a higher value (int var + 1 = new int)
In the example you could use a timeline to open the door (recomended), but you can use other ways like “set actor rotation” over tick
Use a Boolean variable, just have a branch that checks it’s value before the door opens.