Hi Joseph,
From the error code and without details into the setup it is difficult to provide solution, however you can check the troubleshooting steps with regards that I think you might run into some issues related to virtual network configuration and role-based access control (RBAC) permissions.
Also with the error I suspect the subnet you have is not in ready state - The subnet you're using for the load test isn't in the Succeeded state and isn't ready to deploy your load test into it.
Example - az network vnet subnet show -g MyResourceGroup -n MySubnet --vnet-name MyVNet
Can you select another subnet and test?
Hope this helps.
JS
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