Hello @Jing Chen （陈静）
Thank you for reaching out to the Microsoft Q&A platform. Happy to answer your question.
The scale set should have application health monitoring for instances enabled. Health monitoring can be done using either Application Health extension or Load balancer health probes, where only one can be enabled at a time.
I see you have chosen "Load Balancer Probe". Now, before enabling automatic instance repairs policy, ensure that your scale set instances have application endpoint configured to emit the application health status.
To configure health status using Load balancer health probes, see probe up behavior.
Note: Please note a basic SKU load balancer health probe isn't supported with a virtual machine scale set.
Let me know in your comments in case you want to further discuss this issue.
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