This error means that your flat file template has an invalid valid for you the provided required fields. This can also be caused by missing valid values for required fields.
You need to check the product’s template and find the missing or invalid valid values.
1- Go to the channel products and click “Edit Template” for the selected SKU.
2- Check each required field and provide the value by clicking “show possible options” as Amazon requires.
IMPORTANT NOTE: The required fields are usually the ones above the “other_image_url1” field in the flat file template.
3- Select the related product, send product data update to the channel and sync the channel.