For starters mate, if you’re using a Command, then you don’t need to bind IsEnabled, the command implementation should decide this.
Secondly, the binding of a ViewModel to a View tends to happen at a bit of a later stage, so it’s best to also set a default value for the binding, like so
Visibility="{Binding ButtCancel, Converter={StaticResource BoolToVis}, FallbackValue=Hidden}"
Third, as Mike pointed out, ensure that your property is public, since the ViewModel and the View are two separate classes.