In his lifetime, J.R.R. Tolkien published two works of fantasy set in a section of the planet Arda called Middle-Earth: The Hobbit and then its multi-volume sequel The Lord of the Rings. While there are hints of other lands and ages in The Hobbit, it’s really in The Lord of the Rings that it’s decisively revealed that these stories take place at the end of the Third Age of Arda…
Over his life, J.R.R. Tolkien wrote and revised many stories about the First Age. These were collected and edited after his death by his son Christopher, and published in the books The Silmarillion, Unfinished Tales, Beren and Luthien, The Children of Húrin, The Fall of Gondolin, and others. It’s a rich and full mythology, and a television studio could take years to tell those stories.
Amazon has the rights to none of them. The Tolkien estate didn’t sell those. (And Amazon doesn’t have the Hobbit/Lord of the Rings-era stories either.)
What the Tolkien estate sold was the rights to the Second Age, but reportedly not the parts of those stories told in the books primarily about the First Age (the Silmarillion, etc.) At the same time, Amazon cannot contradict those stories either. Amazon’s series will have to be consistent with the Tolkien canon, while at the same time drawing on the vaguest, least detailed portion of it: genealogies, a few outlines of stories, and not much more.