@Xiangrong Xu Thanks for your patience on this. I have summarized our offline conversation and posting the same as answer for the benefit of community members.Issue : App Service Domain creation is getting failed with error "subscription has reached out of quota"
Solution:
Our internal team has investigated and fixed the issue on customer subscription. Post that from the backend logs we can see that customer is able to create the app service domain without any issues.
Hope this helps, let me know if you have any further questions on this.
Please accept as "Yes" if the answer is helpful so that it can help others in the community.