Is it challenging to write a best-selling or a great novel?

Of course it's challenging, or everyone would do it.

For your novel to rise to the top, you have to have a number of things go right. Some of those things are in your control and some aren't.

But, even before that, you have to WRITE THE NOVEL. You have to sit down and get 50,000+ words exactly the way you want them, with no one leaning over your shoulder telling you to get to work. There is no guarantee that it will get published at all, let alone be a bestseller, but you still have to put in all the work.