Customize function not working/changing header logo

I have been trying to replace the logo in the header of my website but can’t access this, when I go to customize the widget areas aren’t showing, it instead says ‘Your theme has 3 widget areas, but this particular page does not display them. You can navigate to other pages on your site while using the Customizer to view and edit the widgets displayed on those pages.’ When going to the widgets through the menu they show as locked. Lost an afternoon to trying to sort this, something which should be very quick and straightforward! Please advise!