Hello, @Faimuddin Shaik (MINDTREE LIMITED) !
Why am I unable to delete subnet which is associated with VMSS?
In an ideal situation, all resources in the resource group would share the same lifespan and you would delete the resource group rather than individual resources. Deleting individual virtual machine scale set (VMSS) resources can be difficult due to dependencies, which is what is causing this error.
In this case, the subnet is in use by the network interface which would need to be deleted prior to attempting to remove the subnet.
I hope this has been helpful! Your feedback is important so please take a moment to accept answers.
If you still have questions, please let us know what is needed in the comments so the question can be answered. Thank you for helping to improve Microsoft Q&A!