If you have uploaded your interior and cover files but are unable to click “Save,” there may not yet be a Shopify Product tied to your Lulu Direct app project. Before you will be able to save your book and sell it in your store, a Shopify Product must be created and connected to the book you are building. Here is an article from Shopify on how to set up a product: https://help.shopify.com/en/manual/products/add-update-products

Once you have created your Product, click "More Actions" and select “Connect with Lulu Direct.” This will take you to the Book Designer page, and you will be guided through the file upload process. Once your files are uploaded and approved, you will then be able to click “Save,” and your book will be ready to list in your Shopify store.