Looking at https://support.squarespace.com/hc/en-us/articles/206540757-Importing-products-from-Etsy-Shopify-and-Big-Cartel, when you utilize the CSV import of Shopify into Squarespace, you cannot add the products directly to an existing store. Instead, it creates a new store under the Not Linked section called Imported Products. This is the Store you must use with your imported products.
If you don't see the store, refresh your page - that was my trouble for a while.