BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6653,
author = {Amila Chandra Kahawalage and M.H. Wathsala N. Jinadasa},
title = {Modelling a Cement Precalciner by Machine Learning Methods},
howpublished = {EasyChair Preprint 6653},
year = {EasyChair, 2021}}