If you already have products added to BigCommerce and you want to sync them with Printful, you'll first need to generate SKUs. These translate as variants on Printful's end and lets us connect your product with a Printful product.
You can read this BigCommerce FAQ for more info on how to create and generate SKUs for your products.
Please note that if you don't have to do this if you add products through Printful's push generator. It's only if you've added products through BigCommerce first.