STM32F446RE Nucleo LD1 slow blinking red and I'm not sure why

Question

I'm using the STM32 using the power from a voltage regulator circuit that generates a stable 3.3V onto the board. Before that, when I tried to upload my program from the Arduino IDE to test it, it went perfectly well with the serial monitor showing the data i needed and all. But when I disconnected it from my laptop and change the power to the external battery, the LD1 on my board was blinking slowly with red. I've tried to read the datasheet and it said:

Slow blinking Red/Off: at power-on before USB initialization

but I'm still having a hard time understanding it because the voltage and the wirings on my board is theoretically correct, and I don't see anything wrong either with the program because it's a simple code as below:

void setup() {
  Serial.begin(9600);  // Ensure this baud rate matches your HC-05's configuration
  
  while (!Serial) {
    ; 
  }

  Serial.println("Bluetooth Test Started");
}

void loop() {
  Serial.println("Hello from STM32F446RE");
  delay(2000);  
}

I would really appreciate the help from anybody.

I've tried resetting my board but nothing seemed to change. I'm just wondering if this is normal or not. If so how can I make my board run normally with an external power supply.