参考文献不能生成对应的序号

编译了参考文献，发现不能显示参考文献序号，序号都是一个类型的。

\documentclass[sensors]{Definitions/mdpi}
\begin{document}
\begin{thebibliography}{999}
% Reference 1
\bibitem[Author1(year)]{ref-journal}
Harle, Robert. A Survey of Indoor Inertial Positioning Systems for Pedestrians. {\em Communications surveys & tutorials} {\bf2013}, {\em 15(3)}, 1281-1293.
\bibitem[Author2(year)]{ref-journal}
Z. Huang, X. Zhu, Y. Lin, L. Xu and Y. Mao. A Novel WIFI-Oriented RSSI Signal Processing Method for Tracking Low-Speed Pedestrians. {\em 2019 5th International Conference on Transportation Information and Safety (ICTIS)} {\bf2019}, {\em pp}, 1018-1023.
\bibitem[Author3(year)]{ref-journal}
J. Pelant et al.BLE device indoor localization based on RSS fingerprinting mapped by propagation modes. {\em 2017 27th International Conference Radioelektronika (RADIOELEKTRONIKA)} {\bf2017}, {\em pp}, 1-5.
% Reference 2
\bibitem[Author4(year)]{ref-url}
DW1000 3.5 - 6.5 GHz Ultra-Wideband (UWB) Transceiver IC with 1 Antenna Port. Available online: https://www.qorvo.com/products/p/DW1000.
\bibitem[Author5(year)]{ref-url}
DWM3000 6.5 & 8.0 GHz Ultra-Wideband (UWB) Module. Available online: https://www.qorvo.com/products/p/DWM3000.
\bibitem[Author6(year)]{ref-url}
REAL-TIME LOCATING SYSTEM FOR SMART INDUSTRY.Available online: https://bespoon.xyz/.
\end{thebibliography}
\end{document}

截图：
我的参考文献显示如图：

别人的：

附属文件：
MDPI_template.zip

报错截图：

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