Hello,
Azure Free Trial credits is valid for 30 days and credits will gets exhausted if not used within 30 days. You can still use the free services but you need to remove the spending limit and upgrade your subscription to Pay-As-You-Go subscription.
You are receiving this error message because you had already activated the free trial subscription and our system wont allow you to create a new one because it is limited to one per new user.
Please note that If you have ever had a free trial on any other account it will restrict you from creating another one in a different account.
Here is the Link for Azure free trial FAQ :
https://azure.microsoft.com/en-us/free/free-account-faq/
Hope that helps !
Regards,
Shubham Agarwal