The problem appears to be in the connections of the devices on the schematics, or maybe a power naming issue. You may find more clues by looking at the netlist for the source to see how the power and ground are connected to the devices in the source. Calibre needs to find power and ground for the devices in order to make the inverter and that seems to be happening ok in the layout but not the source. Another thing you could experiment with is to temporarily use LVS INJECT LOGIC NO (along with LVS RECOGNIZE GATES NONE) so that the devices in this case won't be transformed to an inverter in the layout and then you will see the errors reported in a different way. Sometimes having the errors reported in a different way makes them easier to debug.
Thanks for your reply. As you had pointed out rightly, it was a mistake in the schematic. The error I made was in the subc connection. I had fixed the same somtime back but forgot to update the same in the forum. And now I am not having any issues on that.
Thanks for your attention in this matter.