@Jamiell
Is it possible that you have already delegated a subnet in your VNet for ANF?
See the following information from the documentation:
Create an SMB volume for Azure NetApp Files
The subnet you specify must be delegated to Azure NetApp Files.
If you haven't delegated a subnet, you can click Create new on the Create a Volume page. Then in the Create Subnet page, specify the subnet information, and select Microsoft.NetApp/volumes to delegate the subnet for Azure NetApp Files. In each VNet, only one subnet can be delegated to Azure NetApp Files.
Hope this helps! If you are still facing issues please let me know and we can work with you directly to resolve your error.
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