In order for your Product Display to hold multiple Products (or Variations), you'll have to change the default Product Display [field_product] settings here:
Administration > Structure > Content types > Product display > Manage fields > Product
/admin/structure/types/manage/product-display/fields/field_product
1. Near the bottom of the form, under "Number of values", select "Unlimited" (or a number greater than 1, if you want to limit the number of variations per Product Display).
2. You'll might also want to check "Refer to the referenced products as 'variations' " in the same form if you want them referred to as "Variations".
3. Save settings