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You must have a sku, normally it is a unique number but can be any text, make it suitable to your business. It appears you are in fashion, you may be interested in the managed solution we have just been provided access to.
As SKU can be used any column with special unique identification numbers or numbers&letters;. Maybe you have in your file UPC or MPN provided by your supplier.
In case you are not sure what can be used as SKU, please, attach a few lines of your file so we could see example and advice you on what can be done.