In the book Green Hills of Africa, what is Hemmingway's message to the public, or maybe a point he's trying to make across to us from the read?

This is the book's central narrative and thematic premise - that the act of creation is in many ways similar to the act of a hunt. It is stated directly in the quote taken from page twelve, with less obviously defined reiterations of the theme found in the quotes taken from pages 72, 116 and 284. Pop's advice to the author at the end of chapter ten (to wait, to take it easy) can also be seen as a reference to this thematic idea. The ultimate evocation of this premise, however, can be found in the action of the narrative. Specifically, by setting up the parallel on page twelve and then narrating the action of the hunt in the way he does, the author is essentially suggesting that in pursuit of creation, of art, the writer/artist/hunter must be patient, skilled, versatile, and prepared to take risks. He must, in fact, be willing and/or able to put him/her self at risk in order to win the "trophy" - in the case of the hunter, a pair of kudu horns, a lion's head, or a zebra skin, in the case of the artist a completed, relevant, evocative painting, novel, sculpture or symphony. It's important to note that the author doesn't make this last specific point explicitly clear - it's present by implication in the developed metaphor. For example, the author's determination to obtain the perfect kudu head can be seen metaphorically as paralleling a painter's determination to create the perfect landscape or portrait or a writer's determination to create an effective novel. Meanwhile the author's eventual success at achieving his objective can be seen, metaphorically, as paralleling the successful completion of such a novel.