Please make sure to:
- Download the latest version of the app
- Choose a keto plan
- Use the app in English
Only in these cases, will you see the option to choose net carbs.
Please note:
The calculation of net carbs is only relevant to the US market as their total amount of carbs includes dietary fiber too. That means that carbs in the US = carbs + fiber. 
To calculate net carbs, you subtract the fibers from the carbs > in the US carbs - fiber = net carbs.
In Europe however, the fiber is already subtracted from the total amount of carbs. So in European nutritional data, the total amount of carbs is the same as the total amount of net carbs. That means you do not see any settings/option to choose, as it is irrelevant.