The variation theme specified for SKU [your SKU] is different from the variation theme that already exists for this product in the Amazon catalog. Your variation theme must be identical to, or include, the existing theme [variation theme].
Products that vary by attributes, such as size or color, require a variation relationship and variation theme. The product you are trying to upload already has a variation theme associated with it ("existingTheme" in the error message).
To successfully upload the product, you must use a variation theme that uses or incorporates this "existingTheme." Example: if the product varies by Size and Color ("SizeColor" variation), then you must specify a theme that includes both Size and Color. See Comparison Table for more information about variation themes. See Creating Product Relationships for more information on using variation themes.