ESXi v6.7 can't attach Serial port to VM

I have a new v6.7 ESXi server running on a Dell 990.

The computer has one Serial port.

How do I test to see if ESXi can use it? I do see it under /dev/char/serial/uart0.

I can add it to the VM (powered-off), via ESXi web interface, as a "Use physical serial port", then I select "/dev/char/serial/uart0".

That saves ok.

When powering the VM on, I get "Cannot connect the virtual device serial0 because no corresponding device is available on the host. Do you want to try to connect this virtual device every time you power on the virtual machine?"

Any ideas?

-Brian