We have a lot of low carb and Keto bloggers who use Create, so we totally understand that you likely will want to make use of the Net Carbs and Sugar Alcohols fields. We made this feature available so that it can be utilized by those who want to display those specific nutrient in their posts.
However, you may have noticed that these nutrients aren't automatically calculated for you.
This is because net carbs and sugar alcohols aren’t available via the nutrition API we have with Nutritionix, and they also aren’t officially recognized by Google for the NutritionInformation Schema category. Therefore, this field is optional and not required.
If these nutrients are something you want to display for your audience, they will need to be calculated manually and typed into the corresponding fields in Create.