@Chinmay Sonawane Thanks for reaching out. You are getting this error due to incorrect api version Could you please use the API version as 2023-03-01 Preview in the request url as given below
PATCH https://management.azure.com/subscriptions/{sub-id}/resourceGroups/{RG-name}/providers/Microsoft.ApiManagement/service/{service-name}/backends/{backend-id}?api-version=2023-03-01-preview
I am able to add circuit breaker successfully for my backend please try and let me know incase of further queries, I would be happy to assist you.
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