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Reason for this Error:
The VNET integration feature must set locks on subnets in the Network Resource Provider, we set these locks by putting a structure called a Service Association Link(SAL) onto the Subnet, which is a privileged operation that only Microsoft.Web and some other internal Azure services have access to, this ends up being odd cases where these locks don't get cleaned up. The result is that the customer will have a subnet that cannot be deleted or used because a Service Association Link still exists on the subnet.
You confirmed you were able to delete the resource now with the help of Support Team.
Please let us know if we can be of any further assistance here.
Thanks,
Kapil
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