Hello @max
For the footer widget to appear in the Customizer > Widgets, please once register the footer widgets from your dashboard by going to, Admin Panel > Appearance > Widgets . please once add any one of the widget in the one of the footer widget area. This will make all of the footer option appear in your customizaer.
For more info, kindly refer to the given screen record : https://www.useloom.com/share/41425377e6e04d83a74e5c72bc9bb64c
Hope this helps.
Regards!!