While you are nursing, you don’t have cycles initially, or, more specifically, it takes a while before the first ovulation occurs again (depending on how much of the hormone prolactin is produced). During that time, you generally have to assume you are fertile, because this first ovulation of course takes place BEFORE your first menstruation has started – meaning it happens without you noticing it, because nobody can say for sure when it will happen.
However, trackle needs a complete cycle (meaning from the first day of menstruation to the first day of the next menstruation, with an ovulation measured and recognised by trackle in between) to evaluate your data (and to prepare a prognosis for the next cycle). This very first ovulation can therefore not be detected by trackle.
As soon as you have had your first period while breastfeeding or after weaning and trackle has successfully detected an ovulation, you can use trackle for your contraception as usual.