It always eat well but my weight never gains i look slim, presently my weight is 56kgs age is 27 years?

Food types matter: You should look at the food labels for their energy(caloric) amount. If you want to gain weight, you just have to eat calorie rich foods like milk and milk products(ice cream , milk shakes etc). Meats also provide lots of calories. You like deserts, they are loaded. If you feel fit, this weight is ok, but if you are tired go see your doctor to make sure that your thyroid gland is not overactive.

Not gaining weight: Your condition as you describe it sounds normal and healthy.

Zero sum: The body weight is a zero-sum game in that it depends on how much you take in and how much you expend. Thus if you are very physically active, you will need more food. It is also difficult to gain weight with a vegan diet. If you have intestinal issue, that may be from a parasite. If you are very concerned, then you need to see a doctor to find out .(malabsorption, overactive thyroid etc).