Standing up for Fard prayers is a pillar for those who are able to do so! If you have difficulty bowing down and prostrating but you can stand up, you must pray standing up and you may sit when you want to bow and prostrate.

I started praying sitting at 6 months bcz i find it difficult to do rukoo & sujood. I cudnt even sit on the floor. Somewhere few weeks ago i hv read that some prayers sitting are not accepted bcz of wrong reason. It bothers me & i dont know if I have misunderstood the permissiblity to pray whilst sitting. Im 31weeks now & not sure if i shud do it or not or these are whisphers.