Hi,
You need to be in the correct location in preview, in order for the settings to appear in customizer panel.
1) Go to Appearance -> Customize
2) In your right hand side preview, navigate to a post that shows the sharing bar.
3) On your left, click on Site Wide Options tab. Go to Main Body Design -> Single Post Settings
4) Find the setting -> Display social sharing buttons in your single posts
Uncheck the checkbox and Save Changes.
Remember, in your right hand side preview, you need to be in a post in order to find Main Body Design -> Single Post Settings in the left hand customizer panel.
Thank you