Is it normal that i can't gain weight at all?

I won't jump to the obvious conclusion and say "ectomorph and deal with it", because first of all I've never seen you, so it's hard to judge.

But even IF you are a classic example of ectomorph.. or hard gainer, gaining mass (and a very decent amount of muscle mass) is still very possible. Just prepare yourself that you'll have work and eat harder than everyone else. And it might take longer than it takes everyone else. So be stubborn and patient.

Now, you got the right mental attitude, but that's not all. First of all -home gym is awesome, really. But you need to find a gym that has some good instructors. Not a fatass jerks that were bodybuilders 20 years ago and now all they want is just chill and watch tv while you bust your ass on pointless and wrong routines. But try to find a place that actually has professional people that do guide you. Don't worry about the other people -fuck it, so you're skinny -so what? They were skinny or fat when they started too.
Gym is a place where you come to transform and work on yourself, not to bust out your insane guns and packs and pick up chicks at the aerobic section.

And, since you're not a beginner anymore, you need to concentrate on compound exercises.. YES, the oldschool weightlifting shit, because no matter how many fancy iso-machines they make -they serve a certain purpose, and certainly not yours. Try to work as many muscle groups at once -backsquats, deadlifts, benchpress, military press. And many more... however, don't DIY this stuff. Those are potentially dangerous if you don't keep the right form and technique, so you'll need someone to coach you on this.

Food-wise: the more you lift, the hungrier you'll get. Seriously. There are special carb-blasting diets for hardgainers, but concentrate on work first and the science later.

This is just a tip of an iceberg here, hopefully it helps a bit. If you got any questions, feel free to pm. Good luck!

I'm 24, 5'9" 136-140, depending on what crappy scale I step on. I eat all the time, never gain an ounce.

Ya there are just as many people trying to gain weight then there is that are trying to lose it. Bird is the word!

Yeah, you probably just have a high metabolism. If you're really worried, get yourself tested for tapeworm.

If you eat healthy and exercise-- i wouldn't worry.

You've been blessed with a fast metabolism.. but be careful, i know a lot of people who eat whatever they want just because they won't gain weight (I used to do the same).
There is increasing medical evidence of the downsides of fast metabolisms. On average, those with fast metabolisms die sooner than those with average/slow metabolisms.
A quote from a study on the effect of increased metabolic activity in relation to early death:

“This increased metabolic rate may lead to earlier organ damage (in effect accelerated aging) possibly by accumulation of toxic substances (free radicals) produced with the increase in energy turnover.”

I would ask a doctor what their opinion is on your weight, as i believe BMIs are not always very telling. I'm just above underweight and I still have some fat on me, so clearly things aren't all cut and dry.

You have a good matabilism! Same here when i eat i loose wieght sometimes!

It is normal to have high metabolism. Keep the heart unfold shape and don't worry aboutgainingweight

I NEVER gain weight eever. I am so slim and my fwends r lyk. WOW but its rly weird. I dont work out. But i do alot of exercise. I guess its just how sum pplz body is. Ive learnt 2 deal with it. Try 2 ignor it 4 a while