BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6112,
author = {Mustafa Khalaf Hashem and Ahmed Slim Abbas},
title = {Diagnosing COVID-19 of Lung CT Scan by Using Convolution Neural Network},
howpublished = {EasyChair Preprint 6112},
year = {EasyChair, 2021}}