I've been reading "A short history of Europe" by Simon Jenkins and it mentions that Spain discouraged and at times made enslavement of indigenous Americans illegal.
I did a bit of background reading and it seems that this is based on religious reasons, but I'm not sure I understand what those reasons actually are and why they didn't apply to Africans.
There is always more to say, but you might be interested in this previous question featuring responses from myself, and the unparalleled insights of u/400-Rabbits.