First of all very nice effort.
Feedback::
1.) Make your UI scalable and add anchor points to it.
2.) Change the shooting button from enter key to mouse button.
3.) Improve your enemy AI cause it just sticks to player when player runs away from enemy.
use (nav mesh) for enemy to follow the player and use the MoveTowards() function in the enemy script.
4.) Add Textures to your ground and walls.
5.) Add sway to your weapon and also add recoil animation to it.
6.) Make your map bigger. You can make a terrain for it. It will be more enjoyable.
7.) Take a chill pill. PEACE.
I am not an expert but after adding these things your game will become more awesome.
There are a ton of things which you can add but for now this will be enough.
Learn these things on youtube.
And good luck.