"Write what you know" is one of those pieces of writing advice that is often misused, and misunderstood.
Some people take it way too literally, and not as simple encouragement to add some familiar elements, or drawing on some personal experiences and views to help flesh out your characters, and setting.
I'm fairly certain it's not supposed to mean you should literally only write things you know.
So it might be something we just shouldn't tell new and aspiring writers.
Instead I say:
Get freaky. Get wild. Explore your fantasies. Embrace cliche. Write something you might think is boring. Copy existing stuff you like, or want to like, but with your own twist. Something you would do different that makes it more appealing to you.
Don't worry if you do it badly. Don't try to perfect it. Reach a point where you can say "good enough", and move on. You'll do better next time. And next time after that.
Write a bunch of trash. Get stuff out of your system. Listen to the voices.
Eventually you will reach a point where the words on the page start to match the vision in your head.
But most of all, I will reiterate: Once you reach the point where all you're doing is fine-tuning, just throw it out there, and move on. If you get too stuck, a change of pace could be healthy. It will never be perfect, so it's better to start something new once the old is "good enough". Some stuff might never get there, but that's okay. It's still fine to move on. Put what you've learned into the next project.
Even if you don't think you've learned anything, you have. I promise you. It's just hard to spot incremental changes, and gradual improvements.
So don't be afraid of writing. Don't be afraid of writing badly. If you show it to people, someone will like it. Even if it's just one person, you have made their day a little better.
But you don't have to. You can write only for yourself. It's okay.
So don't be afraid. Just write. Write what you want. Whatever you want.