A virtual network is required when you create a NIC. Select the virtual network for the network interface. You can only assign a network interface to a virtual network that exists in the same subscription and location as the network interface. Once a network interface is created, you cannot change the virtual network it is assigned to. The virtual machine you add the network interface to must also exist in the same location and subscription as the network interface.
Reference:
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-network-network-interface