what should I check on my subaru impreza as it won't engage in D or R?

A few things come to mind. Did you use the correct transmission fluid? Did you replace the proper amount of fluid? Did you replace the filter? And last, sometimes changing the transmission fluid kills it. I've seen it happen in older vehicles.

I hope you didn't flush it. Drain-and-fill is good, a flush does more harm than benefit. Some automatics have a magnet on the bottom of the pan to collect metal 'dust', any metal attracted to a magnet. i.e. steel from gears, and any crap in the filter where it does no harm-- and circulates it through your transmission