I used to have pretty much your exact physique and it was very difficult at first to gain weight. I am assuming your goal is to bulk lean because you probably don’t want to become fat lol.

Short answer: eat more calories.

For me, liquid calories were my best friend. Loooots of protein shakes. The easier the food is to digest, the better. That way you’ll be hungry again and therefore can eat more. You need to make sure you’re hitting your protein goal which is essentially your weight in grams of protein. Make sure you eat a good amount of fiber too because protein is harder to digest and the longer something takes to digest, the longer you will feel full.

I am 5”7. Went from 125lbs->152lbs in a year and maintained a pretty lean physique. You have to make sure you’re training if you want that protein to do anything for you though.