According to the documentation
https://azure.microsoft.com/en-us/free/
After your credit, keep getting free services
Move to pay as you go for free monthly amounts of popular services for 12 months and 40+ other services always.
Azure Kubernetes Service (AKS)
Deploy and manage containers using the tools you choose.
Compute Free
Free period Always
From my understanding - that means - you'll get those $200 for your first 30 days then you change into "pay as you go" and have a monthly budget that can be spend on AKS...
But unfortunately, you have to pay for that service and it is NOT free that's why you cant deploy it.