Virtual Switch … is used by VM virtual network adapters in host…
Disconnect the virtual network adapters from the virtual switch and retry the operation.
You get the above error when trying to remove a Logical Switch from a host. Be sure you want to do this though! This will result in loss of connectivity to VMs and/or hosts depending on what the switch’s being used for. Quick screenshot of error in my lab:
I have active VMs using this Logical Switch. Sure enough:
Either change the switch the VM is connected to or remove the adapter. In short, the error means while the switch is being used VMM wont let you delete it. Fair!