It's okay if... -

You don’t know where your story is going.

You don’t know the ending to your song (or the beginning for that matter).
You don’t know how your plan will come together.
You don’t know if the recipe will work.
Knowing isn’t the point. Creativity is about discovery. It’s about asking interesting questions and searching for solutions (yes, there are many solutions; that’s why you keep creating).
So if you don’t know…that’s okay. You’re not supposed to.