import javafx.application.Application;
import javafx.stage.Stage;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox; //error appears after the semicolon on this line

public class Main extends Application {

    Scene s1, s2;

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage window) {
        Label label1 = new Label("s1");
        Button button1 = new Button("Click for s2");
        button1.setOnAction(e -> window.setScene(s2));

        VBox lay1 = new VBox(50);
        lay1.getChildren().addAll(label1, button1);

        s1 = new Scene(lay1, 500, 500);

        Button button2 = new Button("Click for s1");
        button2.setOnAction(e -> window.setScene(s1));

        s2 = new Scene(lay1, 300, 250);

        window.setScene(s1);
        window.setTitle("title");
        window.show();
    }
}