Be careful of your imports. There are two Button types involved with this. There is android.widget.Button and com.google.appinventor.components.runtime.Button. The latter is the argument to the function, and the other is the thing being operated on. Because they both have the same name, to prevent a collision you will need to reference one of them by the fully qualified class name.
Alternatively, at least for this particular instance you could use the Text() method of the App Inventor Button type to change the text without having to mix the two types.
Evan