Is the prayer valid when one is unable to bend the knees, leading to sitting in a regular posture instead of the sitting between the two prostrations and for the tashahhud?

There is no objection to sitting in the prayer’s sitting positions according to one's ability, and the prayer is valid. The matter is broad regarding the manner of sitting; any posture adopted by the worshiper is sufficient, and their prayer is valid. So, "there is no specific posture required for sitting in these positions for validity; rather, any way one sits is sufficient, whether by tawarruk, iftirash, extending one's legs, raising one or both knees, or anything else. However, the Sunnah is tawarruk in the last part of the prayer and iftirash in other parts."