I found this exact code via GOOGLE Search w/an LG G2 that had also been rooted, but, returned to an unrooted state.
Same issue, could not update, at least via OTA.
LG is detecting that the phone was tampered with & will no longer be updated via OTA. I believe there is a method to restore your device to its out of the box state & be able to update via OTA, look around in the G3 All Things Root section or XDA-DEVELOPERS.
Until then,more than likely, you'll have to use the LG Update Tool/PC Suite for the G3.