Hi @Binit Sahu,
Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.
It appears that the deletion is being blocked on the backend due to the error mentioned above. To resolve this, we will need to submit a support request to remove the subnet delegation and delete the Virtual Network.
To troubleshoot further, we will need a specialized 1:1 session, where a support engineer can have a screen share session and help you resolve your issue. If you have a support plan you may file a support ticket, else please do let us know, we will try and help you get a one-time free technical support.
P.S: We will be able to provide you one-time support if and only if your subscription is not managed by a CSP.
Refer to how create support ticket: How to create an Azure support request - Azure supportability | Microsoft Learn
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Thanks,
Sai Prasanna.