Some Muslims perform their regular daily prayers as well as the Sunnah prayers while they are sitting. However, they are able to stand and walk with no problems. Is their prayer valid?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Standing up is a pillar in the obligatory prayer and the prayer is not valid without it unless one cannot perform it while standing.

Allah Says (what means): {Maintain with care the [obligatory] prayers and [in particular] the middle [i.e. ‘asr] prayer and stand before Allah, devoutly obedient.} [Quran 2: 238]

If the praying person is unable to stand because of an illness, a wound, and the like, then there is no objection to his praying in the state of sitting or in any state that suits his situation.

The evidence for this is the Hadeeth narrated by ‘Imraan Ibn Husayn who said: “I had piles [hemorrhoids], so I asked the Prophet about the prayer and he said: “Pray while standing and if you cannot, pray while sitting, and if you cannot, then pray while lying on your side.” Al-Bukhari

As for the supererogatory prayer, one can perform it in the state of sitting even if he can stand but his reward is half the reward of the person who prays while standing if he has no legal excuse for praying while sitting down.

‘Abdullah Ibn ‘Amru narrated that the Prophet said: “The prayer of one who sits is half of the prayer.” [Muslim]

Besides, ‘Imraan Ibn Husayn narrated that the Prophet said: “The prayer of a standing person is better than his prayer while sitting down, and his prayer in the state of sitting down is half (in reward) his prayer while standing. And his (reward for) prayer lying is half (the reward of) his prayer sitting.” [Ahmad]

Allah knows best.