That’s what you always hear. Write what you know. I only agree a bit on this. I mean you aren’t going to see a murder mystery writer killing someone to see if it works in their book. At least I hope not! I would say it is more true for where and who. You can base characters on people you know, write speech as how you speak and describe places you live or have visited. The rest is imagination or research.
This is why I love writing fantasy. You can use creatures from myth, or make up your own. Where it takes place can be based on an area you know or made up. It can be more imagination then anything.
Fantasy gets a bad rap. People think it’s easy to write, and it really isn’t. It requires just as much thought as any other story.
You can write what you know, it just might work for you. I’m going to keep writing what I know I can imagine.