What is this fruit found in a market in Uyuni, Bolivia at the end of December 2019? (orange skin, 4cm, white flesh)
I found the name: it appears to be Garcinia humilis, known commonly as achachairú or achach:


  Garcinia humilis, known commonly as achachairú or achacha, is a small, prolifically-fruiting tree related to the mangosteen. It grows in the southern part of the Amazon basin in the central area of Bolivia, but has recently been planted on a commercial scale in Burdekin, Australia.


That matches the area, appearance and season.