Is it obligatory to make up the fasting days missed during puberty due to menstruation, knowing that I was not observant of prayer during that period, and does not praying constitute disbelief that negates the need to make up those days?

Prayer is a mighty pillar of Islam, and abandoning it carries a severe warning. Whoever denies its obligation is a disbeliever. As for abandoning it out of laziness, there is a difference of opinion, and the majority do not consider it an act of disbelief. Repentance is obligatory, as is promptly making up missed prayers and fasting. If you do not know their number, continue making them up until you are reasonably certain that you have fulfilled your obligation, and this is the preferred view of the majority. Fasting is forbidden for a menstruating woman, and she must make up the missed fasts. It is obligatory to learn the rulings of prayer and fasting to the best of your ability.