Praying on the chair in permissible ONLY with valid excuse if someone is sick or paralyzed or any excuse that may prevent him from standing.
Narrated by al-Bukhaari (1050) from ‘Imraan ibn Husayn (may Allaah be pleased with him) who said: I had haemorrhoids, and I asked the Prophet (peace and blessings of Allaah be upon him) about praying. He said: “Pray standing; if you cannot, then sitting; and if you cannot, then lying on your side.”
Ibn Qudaamah (may Allaah have mercy on him) said: If standing will make a sick person’s sickness worse, then he should pray sitting. The scholars are unanimously agreed that a person who cannot stand may pray sitting. The Prophet (peace and blessings of Allaah be upon him) said to ‘Imraan ibn Husayn: “Pray standing; if you cannot, then sitting; and if you cannot, then lying on your side.”
Allah knows best