@Moumita Roy Thanks for posting your question in Microsoft Q&A, apologize for any inconvenience caused on this.
If you are leveraging a non-pay-as-you-go offer, example Azure Students for Starter subscription, you may leverage the Free Tier on App Service (Windows or Linux), depending on your requirement.
As mentioned in the similar thread here #1657059 If you want to create app service other than Free tier then you need to raise a support ticket to investigate and assist you further on this.
Hope this helps, let me know if you have any further questions on this.