Panorama greyed out in PSE 11 using Mac Sierra

I have been using my PSE 11 since it was new and have had no trouble creating panoramas.  Now I find that that option is greyed out in the Photomerge selections (all the others are OK) and this is probably the first time I have tried to use the function since moving to Sierra on my Mac.  I am definitely in 'expert' mode.

Any suggestions please?  Does anyone else have this problem?