The Premium SKU for Event Grid Topic is mentioned in various Azure documentation sources, including the Azure CLI and Bicep documentation. However, as you’ve experienced, the Premium SKU is currently unavailable for deployment. This is why you encounter errors when setting the SKU to Premium.
The documentation mentions the Premium SKU and the Azure CLI is in preview, suggesting that Microsoft is planning to introduce this feature in the future. The preview status indicates that the feature is still under development and testing and is not available for general use (yet).
Currently, you can only use the Basic SKU for Event Grid Topics.
Reference:
https://learn.microsoft.com/en-us/cli/azure/eventgrid/topic?view=azure-cli-latest
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