Hello Suraj,
Are you trying to create it thru an ARM template or from Azure GUI?
It seems to complain about a missing identity in the parameters section.
Alternatively, I suggest you using Azure CLI to create an ARO cluster, I tested myself and everything went fine.
You can follow this documentation https://learn.microsoft.com/en-us/azure/openshift/tutorial-create-cluster#create-a-virtual-network-containing-two-empty-subnets from the point of vnet creation with two empty subnets.
Hope it helps to achieve your goal
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