BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6241,
author = {Yong Wang and Peng Tian and Yu Zhou and Mao Mao Zhu and Qing Chen and Chang Zhang},
title = {Using VIVE Tracker to Detect the Trajectory of Mobile Robots},
howpublished = {EasyChair Preprint 6241},
year = {EasyChair, 2021}}