If you are doing multiple "Product Import"  by changing Attribute Set  and getting  "Import Process Failed,Please check errors tab" alert message, navigate to the  "File Preview "  field and open  "Errors"  tab.  

The reason that you receive "Error with Attribute Sets"  is  there are Products with same SKU with different attribute set, so when  importing it throws error on product update.

So if you are doing product import  make sure there is no  duplicate SKU's in the .csv file.

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