@veeman
Take our medieval history time period, add in lots more dangerous things to fight and more ways for people to make money by dangerous expeditions and see what you get.
I mean, slaves as a system sort of stopped existing in Europe when Christianity got a really strong foothold, Vikings might've been the last ones to enslave other europeans, but arab pirates enslaved christians they captured for several hundred years after that (although some did get a choice to convert, die or be captured ).
The whole triangle trade was very profitable, and it benefitted all 3 parties African chiefs who either sold their own people or waged war to capture other tribe's people to sell, Plantation Owners anywhere in the Americas who used them for labor because they were cheap, en masse and could withstand the climate and Europeans who manufactured lots of products from the crops grown, some of which were used to trade with the african chiefs existed until a few hundred years ago.
And depending on the nation, serfs in that specific word or just the sense of it existed for a long time as well, mostly tied to plots of land or aristocratic domains of various sizes. They weren't slaves who could be freely traded, but belonged to the land so if the landlord was traded, they switched owners unless they escaped. The last nation to my knowledge that still had serfdom was Imperial Russia and that only crumbled last century.
So... frankly a civilization at that level, in that world setting not having slavery of any kind, be it crime slaves, debt slaves, war slaves, racial slavery etc. is... utopical. And there usually is some specific reason for it. But even if there is, there will always be someone who tries to create it for their own gain, because to them it's just so profitable.
So the question isn't 'Is there slavery or not?' the question should be 'How socially acceptable and widespread is slavery ?'