How to fix line to line spacing error in Allegro?
Your spacing constraints are being violated probably by that unconnected trace in the bottom left.  Could also be the little jog you have at the end of it.    Go into constraints, physical and see what your spacing is, if those traces are the same net make sure you check same net spacing.   

For more clarity you can turn on drc layer under visibility.  Also if you're really stuck turn on drc layers, and all copper layers.  Run drc report then Zoom way in somewhere.  Now click on coordinates and it will take you right to the problem.

Make sure it's not on some layer that's not visible too