When you can't gain weight it's usually because you have a slow metabolism.
When you have a slow metabolism it makes some people not gain weight even though they are eating a lot.
It can also mean that you're not eating enough calories and need to consume more calories than other people.
Sometimes your health is just fine even though you cannot gain weight as some people are naturally not able to gain weight.
As long as your body is a healthy weight to begin with then there's no need to gain weight and actually that can be a good thing.
When you gain too much weight you can become overweight which is not healthy for you.
Overweight people can suffer from diabetes, heart disease, strokes, heart attacks etc.
It's okay to not be able to gain weight and my brother can eat a lot of meat, fat content etc and never gain any weight and he's healthy.
Me on the other hand I gain weight when eating just a few calories.
So as long as your weight is healthy it's okay to not gain weight.