Stacking Auto-Stack not working

LrC v15.4.1 Windows-11:   I am trying the Auto-Stack ‘by Visual similarity’ on a folder of images with many images I consider similar. 

Why am I getting the message- “Current slider settings doesn’t allow Auto-stacking”? and the [Stack] button is greyed out. Although the previews suggest that stacks will be created.

This message appears for any position of the slider- except for the far right end when [Stack] appears but result is “0 stacks,...”

Something I am missing?