The first thing that comes to mind for me is having her eyes checked. I know in IGs there is PRA and it starts to present as night blindness first.
If there isn't anything wrong with her eyes, or you don't think there is, try making going out to the grass at night something special. Get super yummy treats, like chicken, something better than usual and just for going out at night. Don't even mention going potty, try to get her excited and happy about the treats as you go out or right before and when you get her outside in the grass, praise her, tell her what a super puppy she is and give her good back scratches and the treats of course. Hopefully, you can make going out a fun thing and get her over her fear. Then get her to potty outside.
