The DFPlayer can run on 3.3V or 5V, but it likes 5V much more. So, when the NodeMCU is connected to USB, connect the VIN pin of NodeMCU (5V) with VCC of DFPlayer.
As the NodeMCU is using 3.3V, RX & TX are both 3.3V, so there is normally no need to use a 1K resistor between D6 & dfplayer to reduce digital noise.
