All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Allah, The Exalted, Has Imposed no difficulties on people in religion, since He Said (What means): "and has imposed no difficulties on you in religion." [Al-Hajj]. In addition, Allah Facilitated performing acts of worship in accordance with a person`s capacity because He Said (What means): "On no soul doth God Place a burden greater than it can bear." [Al-Baqarah/286].
Moreover, ‘Imran b. Husain said :I had a fistula; so I asked the prophet (PBUH). He said: offer prayer in standing condition; if you are unable to do so, then in sitting condition: if you are then at your side (i.e, in lying condition).
In conclusion, necessity knows no law and the state of compulsion is assessed according to the extent of the necessity, so whoever is capable to pray standing, he is neither permitted to sit nor to pray on a chair. However, it is worth pointing out that in the past it wasn't common for Muslims to pray on chairs. Nonetheless, this is allowed, nowadays, but in line with the degree of necessity. However, the one who prays on a chair shouldn't pray in the middle of the row in order not to harm or disturb the people praying behind him. Rather, he should pray in the sides of the row or in the back rows. And Allah Knows Best.