Hello Wanling Jiang,
Thanks for your question.
I experienced something similar before.
- Go to the "Subscriptions" blade in the portal.
- Look for any existing Azure subscriptions that might be linked to your Visual Studio benefit. It is possible it is activated in another subscription.
If the Visual Studio subscription was transferred to you, it may have already been activated. In this case you may need to follow steps here: https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/billing-subscription-transfer
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Regards,
Abiola