Replacing Door Sweep in Track from House to Garage?
Some threshold cover strips are replaceable. 

Try to gently pry up on it with a flat bar or putty knife, usually they are pressed into a groove, then take it to you local hardware store and look in the door threshold area to see if you can get a match. If you know the manufacturer of the door you  may be able to contact them and see about a replacement.

Even it you can not replace it, it is not a big deal to have it missing, you can get a door sweep and attach it to the door to cover the gap. You can get the kind, like this, as it can be adjusted down. It appears your door has similar fins that are missing pieces.