Hello,
Customer should leave the review for your product https://gyazo.com/0ef74dbe9fc698876ea23117a90012eb
If you don’t have the review tab on the product page then go to Theme Options > Single Product > Tabs > make sure that tabs are enabled. Then go to Products > edit product > Product data > Advanced > check the Enable reviews http://prntscr.com/oivhjn
Regards