I've seen various references on online forums and websites to a direct flight from Sucre to Uyuni in Bolivia provided by Amaszonas since September 2017. But I cannot verify this. Does a direct flight exist?
Amazonas used to be the airline that did this. However, their route map now shows this is no longer the case. Flight searches on search engines all seem to support this - you have to hop via say, Cochabamba or La Paz.