Is it permissible to pray while sitting due to knee stiffness, and does that affect the validity and acceptance of the prayer?

It is permissible for a sick person to pray while sitting if standing is overtly difficult for them, or if they fear an increase in their illness. They will receive the reward of one who prays standing, for the Prophet, peace and blessings be upon him, said: "If a servant falls ill or travels, what he used to do when healthy and at home will be written for him."