BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6876,
author = {Eslam Alharbi and Mohd Nordin Abdul Rahman},
title = {The Gamification for Elearning in Jordan Higher Education: Literature Review},
howpublished = {EasyChair Preprint 6876},
year = {EasyChair, 2021}}