Install appmenu-gtk2-module by entering the following commands in the terminal:
sudo apt update sudo apt install appmenu-gtk2-module
GtkMenuShell D-Bus exporter (GTK+2.0)
GTK+ module for exporting old-style menus as GMenuModels. . Many applications implement menus as GtkMenuShells and GtkMenuItems and aren't looking to migrate to the newer GMenuModel API. . The AppMenu GTK+ module watches for these types of menus and exports the appropriate GMenuModel implementation. . This package contains the GTK+2.0 AppMenu module.