The lvs.rep.ext file should have details.
(.ext extension added to the LVS report file name for details about warnings related to the connectivity extraction stage)
You can choose if you want the LVS run to halt when this happens (the behavior you see right now) or change the setting so that the LVS continues in spite of the power/ground problem.
In Calibre Interactive GUI the related settings can be seen at the menu choices for:
Setup >> LVS Options >> Supply >> Abort LVS on...