Question

what will happen to person who is not getting enough carbohydrates and proteins

Answer 1

The person will become sick and weak.

Answer 2

body needs  carbohydrates and proteins  I have not seen a study showing directly that harm comes from not enough carbohydrate, there are studies that show eating too little or too much protein can have a downside, and studies showing eating too little or too much fat can have a downside, therefore, you should make sure that you are getting the right amount of protein, and the right amount of fat, and then fill the rest with carbohydrate,So yes, indirectly, there could be a downside to eating too few carbs, but only due to eating too much protein and/or fat while attempting to hit a caloric target.