Shopping cart can be deleted only if it is in "in preparation" status. Once it is requested, follow up purchase request will be created from the cart and so system will not allow you to delete the cart anymore.
You can cancel the shopping cart by cancelling the remaining open quantity of the purchase request created as a follow-on document of the shopping cart.
If there are follow up delivery documents and purchase orders, please cancel them in reverse order, i.e. cancel the delivery document first, then cancel the purchase order and then cancel the open quantity in purchase request at item level using Actions-> Cancel Open Quantity.
This will cancel the shopping cart.
If the purchase order is in preparation status, you can delete it and then cancel open quantity of purchase request.