I have arthritis in my left ankle and cannot bend it to sit on my legs between sajdah, can I kneel on the floor and do sajdah on a chair.

Since you experience difficulty making sajda, you can sit on the chair and
perform your salaah. You do not have to kneel on the floor. For ruku you
should lower your head and for sajda, you should lower it more.

And Allah Ta'ala knows best

Mufti E. Desai