Or, rather, it’s not a GOOD excuse.
Because even people who’ve written books before say they don’t know HOW to write a book. They just do it. And each book is different than the next anyway.
BUT.
If needing to know how to write a book before you try to write one is a belief playing around in your head, then it absolutely is the reason you’re not writing yours. Why? Because your thoughts influence your feelings which dictate your actions which determine your results.
Thinking you don’t know how to write a book (short story, screenplay, sentence) is just that: a thought. And, luckily for you, it’s a thought you can change. But only if you recognize it’s lurking around in your head unwelcome.
The key is to consciously decide what you want to change that belief to. Then, you can cement that new thought into your mind regardless of the reality you’re “seeing” and start to take different actions and get different results.
Only when you change that thought will you actually become the thing you want to be.
(This goes without saying, but change “I don’t know how to write a book and therefore I can’t” to whatever lies your brain tells you that are keeping you from your writing. Actively pay attention to your thoughts—like, write them down—and re-work them into thoughts that will benefit you instead of harming you.)