BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6195,
author = {Herry Sufyan Hadi and Putri Yeni Aisyah and Syamsul Arifin and Ahmad Fauzan Adziimaa and Arief Abdurrakhman},
title = {Fluid Viscosity Measuring Instrument With Internet Of Things (IoT) Based Rotary Method},
howpublished = {EasyChair Preprint 6195},
year = {EasyChair, 2021}}