They do not speak or use any kind of written language
Scientists find the Inca culture odd because they lack a written language. There is no documentation. They did, however, have khipus or quipus, a very sophisticated system of knots that kept track of financial records for their products.
No one has yet figured out how khipus works, although it was usable for encoding history and messages. It doesn’t seem to be a problem we’ll be able to solve any time soon.