Configure virtual network in setting up Windows Virtual Desktop

I am trying to setup Windows Virtual Desktop in Azure.  I'm at the point where I configure the VM settings.  I'm a little confused as to what I am suppose to do when setting up the virtual network and subnet.  What exactly am I doing here?  Am I setting up the virtual machine that connects to Azure AD?  If so, can I just choose the default values that were provided in the fields?  At the end when I tried to complete the whole process I received the below error so it seems like I'm having issues with this part of the setup.  Any advice would be appreciated.  Here was the error below that I received.

Deployment template validation failed: 'The provided value 'Microsoft.WindowsAzure.ResourceStack.Frontdoor.Common.Entities.TemplateGenericProperty`1[Newtonsoft.Json.Linq.JToken]' for the template parameter 'newOrExistingVnet' at line '158' and column '24' is not valid. The parameter value is not part of the allowed value(s): 'existing'.'.