To add widgets in Footer, you have to first add a Widget component into the page.
Open the list of Components and use drag-and-drop to add a Widget component to the footer of the page you are customizing.
To add widgets to an empty widget area, go to Widgets, in the left-hand menu of the Customizer.
Click on Footer Widget Area, then click on Add a Widget:
You’ll expand the list of available widgets. Select the widget you want to include in the Widget area:
You can add one or more widgets to the Widget area you included in the page: