I have a Virtual Thermostat that I created. I have it controlling an exhaust fan in my attic. I set the only supported modes to off and cool. See below.
The thermostat works and I can be updated via a dashboard. This is reminiscent of when the format for the mode string was incorrect, see here. In this case it works fine in the dashboard. When I try to use mode as a condition, the drop down to select cool or off will not allow me to select it. Can you not limit the modes of a thermostat?
[edit]I think I found the issue. You have to have heat as a mode because apparently the default for the drop down when using mode as a condition is heat. If heat is not an option, it doesn't like it.