Is the Hajj valid if the fetus was miscarried in the second month, and dark brown discharge (not blood) appeared on the eve of the Day of Tarwiyah after purification?

The discharge you saw is not menstrual blood or postpartum bleeding; rather, it is a "turbid discharge" (kudra) that does not affect the validity of Hajj or Umrah. According to the majority of scholars, one must perform ablution (wudu) for it. If you performed ablution before the Farewell Tawaf, then your tawaf is valid by consensus. However, if you did not perform ablution, scholars differ on the validity of the tawaf. Ibn Taymiyyah and Ibn Uthaymeen chose the view that purity is not a prerequisite. Turbid discharge and yellowish discharge after purification are not considered menstruation and do not prevent prayer, fasting, or intercourse. Your Hajj is valid, Allah willing.