BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6787,
author = {Ioana-Ancuta Halmaciu and Ioana Ionel and Mihail Reinhold Wächter and Ion Vetres},
title = {Biogas from Poultry Waste - a Source of Energy},
howpublished = {EasyChair Preprint 6787},
year = {EasyChair, 2021}}