Assuming you mean the "Eurotronic Spirit Thermostat" driver, this one looks like it populates the supportedThermostatFanModes attribute if the device responds with this information (like most Z-Wave Plus devices do). I'm guessing this one doesn't given its nature. That may be something that can be addressed in the driver.
In the meantime: you can fix this error by adding the attribute yourself. Temporarily switch to the Virtual Thermostat driver. Then, put something like this in the parameter textbox for the setSupportedThermostatFanMode() command:
["auto"]
(or whatever JSON list you want this to display--with no fan, I assume you don't particularly care, but it needs to be there.)
Then hit the button to run the command, switch back to your original driver, and the Dashboard problem should be fixed.
Well, using less than 20 buttons on a virtual device (say, using multiple devices instead with a few buttons each) would be one way to solve that problem, then. 
But people have also asked for the ability to "label" buttons on a dashboard with custom text instead of the default button number. If/when that feature is implemented, then that may help you.
It may be helpful to keep in mind that Easy Dashboard is new, and the developers are listening to feedback. Some changes and feature additions have been made based on requests. It is new, and more is likely to come in the future. But it is called "Easy Dashboard," and not everything everyone requests can possibly be implemented. Hubitat Dashboard is not going away (and will probably always be more customizable), so if that meets your needs better, there is no reason you cannot continue using it.
