Thank you for your reply. I figured out the problem to this particular issue and will share my conclusion for the benefit of the community. I exported a full product (meaning every product table) CSV from the admin panel. Thus there were multiple "product_id" columns in the spreadsheet, ordered as they appeared in certain tables (i.e. _product_description table has a product_id column, as does _product_categories, and so on). The real mistake I made was filling in the product_id column that was part of the "products to categories" table with our id numbers, but not having a associated category id to all of the product ids listed. Hence the error message. I went back and corrected this and the upload was successful, even though prices and dates were off because I used Excel. Trial and error, lol.