why does the automatic transmission not engage?

on 2001 Ford Explorer Sport Trac

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
Nothing happens when it goes from Neutral to Drive or Reverse. It is like it does not engage. It has a dip stick. Where is the fill cap located?
(1) Answer
You are right to check the transmission fluid level first, and the fill point is the dipstick tube. Make sure to check with the engine running, and if low, add the appropriate fluid according to your owners manual.
If adding fluid does not help, you may have a torque converter or internal transmission failure on your hands.