While upgrading "/system/installation/packages/osinstall.mpkg appears to be missing" message pops up and not able to proceed further

I got an upgrade note from apple on my mac book pro and I proceed with the steps. After restart I keep getting "/system/installation/packages/osinstall.mpkg appears to be missing" pop up window and not able to proceed further. Even after restart I keep getting the same message. I am stuck here and not able to work on my MAC. Please help me how do I get rid of this message? and fall back to my previous version of OS.