I think 'write what you know' isn't just about avoiding mistakes—although that does count if your fiction is set in the real world, past or present. If you 'know' something well, you will be able to come up with details that are convincing and bring the reader on board. If you've actually been to a place, or done a certain job, you'll 'know' more than somebody who just researches it. You'll know what the wind smells and feels like in the desert, or what happens when you flip a burger on a large commercial grill, or what the noise surrounding you as you work in a newspaper office is actually like. However, 'what you know' can certainly be added to by research. Many MANY great writers do range far outwith their own experiences. The quality of the images and events they create boils down to the quality of the writer's imagination. However imaginative you are, though, you will certainly screw up if you don't know what you're talking about. Unless you are writing pure fantasy, be careful to do some research if you're moving away from familiar ground. Sure as tooting, somebody who does know about the subject will be attracted to your book because OF the subject ...and if you make too many mistakes, your reader will lose faith in you. I don't think anybody's writing was ever ruined by research ...unless they fall into the tempting trap of showing off how much of it they've done. Research should underpin your writing. It should never show!
I am asking myself do I belong here. I now belief in Creative Writing. All I need is a subject then write what you know and feel. It makes sense. but I have other serious issues to tend to first, my emotional overload has too be fixed now. Later. Writer one