There is a class of things that people understand takes practice, which includes things like skateboarding, playing the violin, speaking a foreign language, and so on.
There is another class of things that people think don't require practice. They think that the first few times they try them, they will get a good idea of whether or not they have a talent for it. This class includes drawing and writing fiction. They are wrong.
To expect that your first short story is going to be good is like expecting your first time playing the violin is going to sound decent, or your first few times on a skateboard halfpipe are going to work.
The truth is that writing, drawing, improv, and a whole lot of things requires practice. Nobody is born a good writer. You need to write or draw a lot to get any good.