Hello @Huan Duong ,
Thank you for reaching out. If you are facing issues when activating Microsoft for Startups Azure Subscription, then I would request you to please raise an Azure Billing Support Request from Azure Portal. Billing team should be able to help you with entitlement validation and activation of subscription on your account.
For more details around how to raise a support request, please review following article: https://learn.microsoft.com/en-us/azure/azure-portal/supportability/how-to-create-azure-support-request
I hope this helps. Please "Accept the answer" if the information helped you. This will help us and others in the community as well.