Configuring requires putting together your options for printing.
In this case it means to select your print and design options.
If a 'configuration is not valid' or 'cover configuration is not valid' message is shown on the basket page, this means you haven't selected your printing options. Please click 'View/Edit' on the relevant item in your basket, to proceed with your product's configuration.
You must then ensure each configuration step is complete before proceeding to checkout.
For paperback and hardback books, a cover is required. Therefore, if left unconfigured, you'll receive the 'cover configuration is not valid' sign on the basket item and you won't be able to proceed to the checkout.