Hi hi! Y'know, I have absolutely never had this problem. I have far, far too many ideas swimming about in my head.
I usually start with characters, because people are fascinating. What would a stubborn person do if holding on to their beliefs causes them physical pain? What if it hurts the people they care about the most, or if one idea they've held their whole life conflicts with something else they believe to be true?
What if an altruistic person has to deal with an irl trolley problem? What happens when a little boy with a vivid imagination suddenly finds that all of his dreams are true and it's the real world that's fake?
The possibilities, they're endless.
But if that's not your jam, settings are also a good place to start. Imagine the best world you can, then think of the problems that would logically arise in such a scenario. Ruin paradise. Or what if one small thing about the world was different? How would that change lives?
Don't write because you want to have any story. Write because you want a specific story. It's already hard enough to finish something you're truly passionate about, let alone something that you couldn't care less about.
Or you can write about yourself. Write a journal entry in narrative form just to get the hang of writing and work on/develop/create your skills.
If all else fails, scroll through writing prompts and write a short story. Then think about what you've written and ask questions. How did they get to that point? What happens next? How would those characters react in an entirely different scenario?
Mess around a bit until something really strikes your fancy.
And... feel free to message me with any story/site related questions or comments. Bounce story ideas off of me. Please don't abuse that, though. While I'm happy to encourage productivity and growth in your writing ability, I am not signing up to be your therapist. Putting that out there now, just in case.