@Jyy Syy Thank you for posting your question in Microsoft Q&A, apologize for any inconvenience caused on this.
If you are seeing the issue "The subscription is not allowed to create or update the serverfarm" -If you haven't done retried, please retry creating the app service again (similar thread #1535067 user was able to fix the problem).
Update:
Our Internal team has investigated and fixed the issue on the customer subscription(cd---b) and customer has confirmed that they able to create the app service.
If anyone is facing the issue, for a detailed investigation, we request you to create a support ticket. Please create a support ticket for an immediate and focused assistance.
Hope this helps and let me know if you have any further questions on this.