Please follow the steps for adding variants in the Back Office

Login to Back Office > go to ‘Product’ from the right menu > select ‘Variants’ option at the top menu > click on ‘New variant group’ > new window will appear on the screen.

Enter ‘Variant Group name’, select store in the drop down option of ‘Store assign’ and ‘Variant value’ > click on ‘Save Variant’ variant will be created.