Below is a list of common validation errors and solutions from Amazon:

Validation ErrorSolution
You are not authorized to list products under this brand
Some categories require approval from Amazon before you can list.  For more information -  Authorisation
Missing Attributes standard_product_id.
  • There is no barcode (UPC/EAN) associated with the product or one of the variants . Add the relevant barcode and resubmit.
  • The barcode you have provided is not recognised by Amazon. Please check it is the correct registered barcode for your product.
The SKU data provided conflicts with the Amazon catalog.
The data provided by the retailer is conflicting with what Amazon has in the database. For example  (Merchant: '' / Amazon: '16192'). The retailer can update their content to match Amazon OR petition Amazon to change the data.
A value is required for the "part_number" field
MPN is required
You cannot change the 'item_classification' for SKU 'XXX' by submitting a feed with a new value. To change this attribute, do the following: 1) Submit a feed to delete the existing SKU. 2) Submit a feed to recreate the SKU with the updated 'item_classification'.;
This product classification has changed from a multi variant / single variant OR single variant / multi variant. For more information  - Change variant
Marketplace Product Variant SKUs are not unique
This product variants are duplicated.  Variant attributes need to be unique  for example colour black, size small etc
Please provide a variation theme when creating a variation parent.
The variation used for a product is not yet configured for that category in Omnivore.  Omnivore Amazon integration is currently only configured to support colour and size. 
UPC/EAN is too short/long
The barcode UPC is missing characters or contains too many.  Please check the UPC against that product.
The "Message/Product/DescriptionData/MerchantShippingGroupName" (XXX) field contains an invalid value.
This indicates that the shipping policy in Omnivore does not match the naming convention in Amazon.  The shipping template name must be replicated exactly and is case sensitive.
Cannot serialise to Amazon Australia feed format, error = Cannot find the Amazon product type
The category is not yet available on Amazon Australia
A value is required for 'department_field'
There is a required attribute for a specific category / sub category. Add in the Attribute and resubmit the feed.