Is it permissible to supplicate for a sick person for a long life and complete health with certainty of a response, and is seeing their death in a dream an indication that the supplication will not be answered?

It is permissible to pray for a long life, just as the Prophet, peace and blessings be upon him, prayed for Anas ibn Malik. It is also legislated to supplicate for the sick for healing, health, and a long life, and healing is hoped for if the conditions for the acceptance of supplication are met, such as supplicating seven times for the sick person whose term has not yet come: "I ask Allah, the Almighty, Lord of the Magnificent Throne, to heal you." As for visions, they are not our specialty.