Hi Sourabh Bansal,
It seems you are experiencing an issue with signing up for the student benefits and receiving the $100 credit on the portal. Here is an overview of the issue along with the steps to resolve it.
You mentioned that the portal says you are not eligible to sign up despite being over 18 and having a valid student ID. This could be due to a mismatch in the information provided or a technical glitch on the portal.
As per the message that indicating your student ID has already been used this year suggests that an Azure for Students subscription has been activated using your credentials. Each student is allowed only one subscription per year.
Check if you have previously signed up for Azure for Students. If so, you might have an active subscription. To renew your subscription after one year, sign up again for the offer as long as you are still a student, Follow this reference Microsoft documentation link: https://learn.microsoft.com/en-us/azure/education-hub/faq
Refer to the below documents for better understanding:
https://learn.microsoft.com/en-us/azure/education-hub/about-azure-for-students
https://azure.microsoft.com/en-gb/pricing/offers/ms-azr-0170p/
If the answer is helpful, please click Accept Answer and kindly upvote it so that other people who faces similar issue may get benefitted from it.
Let me know if you have any further Queries.