Correct Camera Distortion in Photoshop Elements 15 for Mac

I'm using Photoshop Elements 15 on an iMac running macOS Sierra, Ver 10.12.2.  I just merged two photos using Panorama, flattened the layers, and tried to use Filter -> Correct Camera Distortion.  The Correct Camera Distortion box opens, but none of the tools work (Vertical, Horizontal, etc.). I have used the distortion tools successfully before on other Panorama merged photos and cannot figure out why they've stopped working for me.  Any ideas on the cause will be appreciated.