After giving birth, you can only use trackle again for your contraception as soon as you have had your first menstrual period (regardless of whether you are breastfeeding or not). Before that, you must switch to an alternative contraceptive.
This is because fertility must always be assumed after childbirth, as your first ovulation takes place BEFORE your first menstruation has started again. In other words, unnoticed, because nobody can say exactly when it will take place.
However, trackle needs a complete cycle (i.e. from the first day of menstruation to the first day of the next menstruation - with an ovulation measured and recognized by trackle in between) for the evaluation (and also for predictions for the next cycle). So trackle cannot recognize this very first ovulation.
If you are breastfeeding, it may take a little longer for your first menstrual period to start - depending on the hormone prolactin. Here, too, you will have to wait until your first menstruation occurs before you start using trackle and fertility must generally be assumed before then.
As soon as you have had your first period (of the usual intensity) while breastfeeding or after weaning, you can use trackle as usual.