Expanding on Elizabeth’s post with an analogy of my own.
First, some advice. Write whatever you want. Flash, short stories, novels, whatever.
Having said that, there’s merit at trying your hand at both. I’m a little biased, maybe, because I’m a short story writer from way back. They don’t intimidate me the way novels do. I love that I can have an idea for a story on Sunday and have a submission-ready draft by Saturday. They’re a different animal than novels, sure, but they use a lot of the same skills. And like Elizabeth said, you get to practice beginnings and endings more.
It’s like knitting. Sure, knitting a potholder or scarf isn’t much different than knitting a sweater. It’s knitting and maybe some purling. But sweaters require a lot of skills that scarves don’t necessarily need. Increasing, decreasing, yarn overs, measuring gauge, knitting in the round, etc. Can you make a scarf with those skills? Absolutely. But to make a scarf you really only need to know how to cast on, bind off, and knit. You can’t make a sweater that way.
I said all that to say this: writing short stories is a great way to get really good at skills you’ll need for novels. Do you have to write them? No. If short stories make you want to give up writing, then absolutely stay away from them. There’s a lot of “rules” in writing, but in my book there’s only one hard-and-fast one: write what makes you want to write more.