Hi Guangjie,
I tried these steps and could create the cron job with your cron.xml config successfully.
Note that your Servlet URL mapping in web.xml should be the same as the cron job URL. If this mapping hasn’t been done properly, you won’t see the cron job even if you deploy your app successfully.
I recommend to double check the steps you have taken with the mentioned documentation. If the issue persists, you may post your full detailed question to Stack Overflow, using the supported Cloud tags since Google Groups are reserved for general Google Cloud Platform-end product discussions and not for technical questions.