What is the ruling regarding a woman praying alone in the women's section of a mosque if congregational prayer has been established but she does not pray with the Imam?

Your prayer is valid, and there is no sin upon you, for congregational prayer is not obligatory for women. It is preferable, however, for you to pray with the Imam to attain the reward of congregational prayer. There is no harm in you standing alone, as it is permissible for an individual to pray behind a row when there is an excuse. Umm Sulaym indeed prayed behind the Prophet, peace and blessings be upon him. The dislike of a woman praying behind a strange man applies to situations in the home or similar places, not in the mosque, especially if there are other men praying with him.