If she was an outdoor cat, maybe she never learned to use a box.

Use the "training the kitten method": Set her gently in the box, and take her front paws to make a scratching motion, to mimic the covering-up part. You may have to do this a couple of times.

Talk nicely to her when you do this, encouragingly, and give her a treat afterwards...and when she DOES use the box, praise her a bit.

Hope this helps!