If you don’t have any periods at all, unfortunately, the Natural Cycles algorithm would not be able to analyze your cycle, meaning that you cannot use the app.
Period data is one of the key indicators that the algorithm uses to calculate your fertility status, since the period marks the start of each new cycle. Without any period data, the algorithm would not be able to identify your fertile window, this means that, sadly, Natural Cycles may not be the best option for you at the moment.
You can read more about how Natural Cycles works here, and you will find more information on Natural Cycles and the menstrual cycle here.