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It is not possible to add VMs in a Standard Availability Set to a Load Balancer in Basic SKU. Azure requires that the SKUs of the Load Balancer and the associated resources, such as Public IP addresses and Availability Sets, match. Therefore, you cannot mix Basic SKU resources with Standard SKU resources. For new designs, it is recommended to use Standard SKU resources to ensure compatibility and take advantage of the enhanced features and capabilities.
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Deepanshu