2012 ford focus not shifting in reverse
Most frequent cause for this is low transmission fluid; check it. If fluid is OK, it might be loss of fluid pressure due to wear or an internal defect. Might be a dirty selector switch. Make several rapid changes from park, through reverse and to neutral; this might free it up. Or it might be damaged linkage if it feels like shifts into gears are not precise.
I use Motorcraft filter (FL 910 S) I have the same year model and I have 166,000 miles on it. I change it every 5000 miles and it used about half a quart of oil at each change.