Hi there,
Marketer was updated to use Block Elements for certain template parts including the Site Footer.
You can now edit the Block Element – Site Footer in Appearance > Elements ( or use the Admin Bar > Elements menu to shortcut to any element on the viewed page ).
This Element replaces the themes footer.
If you don’t want to use the Element, then delete it ( or Quick Edit > Change Status to Draft ) and then the Customizer method can be used.