KiCAD - plethora of errors when performing ERC
Looking at the circuit and where the ECR arrows are pointing I can see two type of violations

undriven net
Power flag

P1, P3, P4, P5, P6, P7 have pins that do not connect to anything. You are obviously labeling them to align to the Mega but from the perspective of this circuit they are unconnected. The intent of this ECR is to ensure you don't leave unconnected nets ... unconnected.  If you intend to leave it unconnected us the &quot;no connect&quot; flag at the end of the net (the blue cross)
You have a couple of global powerflags (3v3, 5v, GND) but KiCAD does not know these are power, use the power_flag to inform kicad this is a powernet
Dangling nets:
if you are 100% sure a net is to be unconnected, add the NC to each one

Power-Nodes
if you are using power symbols that do connect to a pin that expects POWER-IN, inform Kicad that this is a power source