Install gir1.2-mate-panel by entering the following commands in the terminal:
sudo apt update sudo apt install gir1.2-mate-panel
Description:
GObject introspection data for libmate-panel-applet4-1 (transitional package)
This package is a transitional package to gir1.2-matepanelapplet-4.0. The package can be safely removed.
Homepage: http://www.mate-desktop.org/
Version: 1.20.1-3ubuntu1
Section: universe/introspection