Why is Savitar face burnt?
Savitar’s face is half-burned because his future self scarred him when Barry and his time remnants fought Savitar in the future.
How did future flash get his scar?
The scar on his face is due to burns. The future flash i.e. Savitar runs so fast that he is invisible for other non-speedsters. This tremendous speed makes a lot of friction, causing burning and leading to scars.