Hey, This is pretty easy to resolve if you are still running into this problem. The problem comes in if you created the Network and Subnet in the same browser session that you plan to use to create the Windows Virtual Desktop pool. Open a browser Incognito and attempt to create the WVD. You should see success. You'll know you'll get success if you can finally see and select the network and subnet you was the WVD to connect to.
This is what solve my exact same issue.