is colname a parameter in the Recordset?
If so it does not need to be surrounded in quotes, when configuring the parameter, set the data type to text.
also the perenthises around the join are not needed:
FROM (Category LEFT JOIN Product ON Product.Product_cat=Category.categoryID)
can be:
FROM Category LEFT JOIN Product ON Product.Product_cat=Category.categoryID
If all else fails, remove the where clause, insert the add to cart button, then add the where clause back in.