Hi Alain -
I need a screenshot of the mosaic dataset footprints, I think the screenshot you sent are building footprints.
What is your data source? Aerial, Satellite or Drone? Note that you need to have at least 50-60% overlap between images to get a stereo pair. And you should be working with imagery that contains sensor model information (preferably that has run through the ortho mapping process). Stereo's and elevation models cannot be generated on preprocessed orthos.