Having a sub-attribute of a shipping error can drive you crazy if you have looked everywhere and you think you have done everything correct. There are two areas you can submit shipping prices, these are your data feed and the Google Merchant Center UI.
The shipping error mostly comes from merchants implementing shipping settings in the UI but incorrectly, or submitting an invalid value.
Below a list of the most common and possible issues, you may encounter.
In some cases you may have indeed submitted a shipping price value, but incorrectly. For example you have submitted $0.00 instead of 0.00 USD
More info on the correct shipping attribute values: https://support.google.com/merchants/answer/6324484
In some cases you would setup shipping prices with a minimum order value. This means only products that are above a certain price point have the specific shipping settings applied to.
In Google Merchant Center every product you list, must have a shipping price.