There is no associated product found for Configurable Product

You get this error because the eBay item is linked to the Magento Configurable Product, but there's no association between options in Magento and variations purchased by the buyer.

So you should manually resolve this situation by using the Set Options functionality for M2E Pro M1 and for M2E Pro M2.

Set the product options, then choose if you want to use the same details for future orders by checking the Save selection for subsequent orders box. Click Confirm to save your settings.

M2E Pro will apply your option linking settings to all new orders with the same eBay item and variation in order.