May Allah make it easy for you.

Yes, you can sit on a chair and pray as long as sitting on the ground hurts you. As long as you can pray standing up, this is a Pillar of prayer if youa re able to do that. If not, you can sit on a chair.

Therefore, as long as you only sit for tasha’hud, this is permissible and your prayer is correct.

I have hurt my colateral ligament (the bone behind the has twisted in the injury) and i can not sit in tashahud becaus my knee hurts, untill i get well is it a sin to sit on a chair only in tashahud.becaus some say to me that i have to sit just like the prophet Muhammed (May Allahs Peace and Blessing Be Upon Him) did and some say that you can use a chair if you are unable to sit or becaus of and injury, so that is my question: if i can sit on a chair only for tashahud?