Deleting a NIC

If you want to delete a Network Interface (NIC) from a Virtual Machine with multiple NICs assigned, make sure that the default gateway is NOT associated with the NIC you're deleting.

If you try to delete a NIC which is the one associated with the default gateway, you wont be able to "Save" unless you press "Cancel"

Isntead you need to go back to the VM configuration panel on the Networks tab, set the default gateway to one associated with another NIC, save, then you can delete the NIC that's now not associated with the gateway.