Must one make up prayers missed due to a misunderstanding of when ritual purity from menstruation is attained?

Your omission of prayer, based on the assumption that you were still menstruating, necessitates making up the prayers according to the majority of scholars. However, Sheikh al-Islam Ibn Taymiyyah and a group of scholars are of the view that you are not required to make them up because you were ignorant. The more cautious approach is the opinion of the majority. The two signs of purity are: dryness or the white discharge.