The error "no translation to external ID <category_name>" appears in the error log when the main category that the item was assigned has not been created or for some reason has been deleted.
[ <category_name> stands for the name you have in your file]
Main Category and Item Code are both mandatory fields required to uniquely identify each item.
(For instructions on how to access the error log see this article: Accessing the Admin Error Log)
Creating the missing Main Category again is simple.
Go to the Main Category (or Brand) tab under Items->Manage Items and follow these instructions, and then re-upload your items.
For more information on Main Categories and Brands: Main Category Tab