Passing a single class instance to another class

[PieterP]

_debug is a reference, not a pointer:

@hendr1x

Box::Box(Debug &debug) :
 _debug(&debug) {              // <----- remove ampersand here
  pinMode(28, INPUT_PULLUP);
}

Your constructor expects a reference, not a pointer:

@hendr1x

void Puzzle::setup(){
  _debug = Debug();
  _box = Box(&_debug);         // <----- remove ampersand here

  
  _debug.line("Puzzle Setup");
}