Thank you for that screenshot! It looks like you are currently editing your Home page. In order to edit your product page you will want to switch the template you are editing to your Default product page template. You can do this by clicking the dropdown menu at the top of the page in your theme editor. Then you will want to select Product > Default product and the Product information section will appear on the left. I’ve included an example below of what this will look like if you are using the theme, Craft.
You can also read more about using the theme editor on this page here.
Don’t hesitate to reply back if you are still having trouble adding the variant picker.