The variation theme specified for SKU [your SKU] is different from the variation theme that already exists for this product in the Amazon catalogue. Your variation theme must be identical to, or include, the existing theme [variation theme].
Products that vary by attributes, such as size or colour, require a variation relationship and variation theme. The product that 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’. For example: if the product varies by Size and Colour (‘SizeColour’ variation), then you must specify a theme that includes both Size and Colour. Refer to the Comparison Table for more information about variation themes. Refer to Creating Product Relationships for more information about using variation themes.