if is_on_floor():
jumps_available = 2
That part will never be executed, since it's within this other block:
if Input.is_action_just_pressed("jump") and jumps_available > 0:
If jumps are 0, then it's not going to enter this block and refill them, since you are requiring the jumps to be above 0.
I think what you meant to do is this:
if Input.is_action_just_pressed("jump") and jumps_available > 0:
velocity.y -= 250
jumps_available -= 1
if is_on_floor():
jumps_available = 2
(Notice the indentation change)