I have problems like very poor digestion and skin psoriasis. my weight was only 48kg at age of 21. how can i gain weight?

Problem: Sounds like you have serious issues that need to be attended to by a competent physician. If you are living in another country or recently moved the the United States and weigh only 48 pounds at your current age, then maybe you have a worm in the intestines not allowing you to absorb any of the food you are eating. See a doctor is the only recommendation.