You need to separate audio from the video file, otherwise it won't work. Separate via ungroup, and see if you can move the audio track without the video moving, just to be sure. Pop the cursor on the video track, right mouse button click, choose Video effects. Another window opens, and in play speed beneath the actual play speed indicator,  you will see an arrow.  Click on that, and the usual setting of 1.0 should now read -1.0. and ergo now play backwards.

If it doesn't allow you to change direction, then you've not separated audio and video