Not knowing your technology nor your DRC rules, I 'd assume:
You have N_WELLs WITH DIFFERENT POTENTIAL in your layout. Seems - in your technology - that N-Wells have to be marked (e.g.) by a DNW_LV_MARK marking layer. But there must be different DNW_LV_MARK marking layer polygons for N_WELLs WITH DIFFERENT POTENTIAL. And these have to be separated by a distance corresponding to the DRC rules.