A difficult question.
First, I think "writing fiction" is not a monolithic activity. For example, some people are natural story tellers, but have to learn how to formulate sentences. Doing what you're good at is fun (and you'll improve quicker, because you'll do it more). Doing what you're not good at isn't fun, but you can be motivated to do it so you get to do that other thing you're good at. So if you have jerky sentences, but your stories are gripping, you'll be motivated to learn how to improve your sentences, because you're stories will come across better.
Now, if you're neither good at story, nor at character, nor at language, nor... you could in theory still learn it all, but I have a hard time seeing where the motivation would come from.
For example, I'm quite aware that while I'm good with character and dialogue I'm... not so good with setting. Writing setting, for me, is slow and awkward, but I'm learning, and it's a revelation to see how the improved setting enhances the characters. If I were not good at character in the first place, I doubt I'd feel compelled to learn about setting.
So yes, I do think, that there are people who just haven't got "it"; i.e. people who have no writing-related strength. But I also think that these people rarely even try to write.
It's not something I find helpful to say at a writing site. I don't expect to find people who haven't got "it", there. Of course, there's evaluation to take into account. If your style tends more towards 19th Century telling, people may tell you you're hopeless. That's nonsense. You're style's just not currently in fashion. (Though there's surely a niche, even if your writing ends up for free on a web-page with about ten regular visitors.)
I tend to sit somewhere on the sidelines. I neither buy the hard-work ethics, nor do I believe in the "natural born writer". If you do what you enjoy, and do it often, you're going to get better (just hope you don't get stuck in a rut).
And that is that.