Write what you know. The only viewpoint you can truly know is your own. Your characters must be a clone-army of self-inserts.
Write what you know. You can’t know about things that don’t exist in the real world. No sci-fi or fantasy.
Write what you know. If something never happened, you can’t know about it. No fiction.
Write what you know. Philosophically it’s impossible to know anything with absolute certainty. No writing.
Write what you know. Since you can’t know anything, you also can’t know whether “write what you know” is even a rule worth following. Fuck it. Write whatever.