Making DLL.

This is the problem, When I copy paste it to the dll and try to build a get a error. It says it failed and ogot 0 succsed and 1 failed. I just copy paste this and it dont work.
void DrawBox(UCanvas* Canvas, APawn* Target, FColor color)
{
if(Target != NULL && Canvas != NULL)
{
FBox Box;
Target->GetComponentsBoundingBox(&Box);

FVector vpjMax = WorldToScreen(Canvas, Box.Max);
FVector vpjMin = WorldToScreen(Canvas, Box.Min);
FVector vpjCenter = WorldToScreen(Canvas, Target->Location);

float flWidth = fabs((vpjMax.Y - vpjMin.Y) / 2);

Canvas->CurX = vpjCenter.X - flWidth / 2;
Canvas->CurY = vpjMax.Y;
Canvas->DrawColor = color;
Canvas->DrawBox(flWidth, flWidth * 2);
}
}

How do I learn it? Please help me.

I got a brain, I got no book. Where can I read it? nad whu u so mad :/