Based on the one photo it appears the gap is relatively even across the full width -- there's no high or low spot preventing closure. Make sure you have some kind of seal installed on the bottom edge of the door. A bulb style like the one in JACK's answer is great.
It sounds as if you've already tried to adjust the door operator's range of travel but it is already at its limit. Inspect the door, the jamb, the rollers, the arm that attaches the door to the operator trolley, and the trolley position. Figure out what's preventing the door from closing further. Is a roller or roller bracket running into something? Is the operator trolley hitting a stop at the end of its rail?
If the trolley position is the limiting factor you could try disconnecting the door arm from the trolley. If the door can close fully when disconnected from the operator then adjust that linkage/arm so it's a little longer. The travel limits of the operator will have to be adjusted again after this is done.