Unable to send data via bluetooth to Feather 32uf board.

This is my arduino code:

String voice; 
void setup() {
pinMode(13, OUTPUT);  
void loop() {
  while (Serial.available())
  char c = Serial.read(); 
  voice += c; 
  if (voice.length() > 0)
       if(voice == "on")
         digitalWrite(13, HIGH);
  if(voice == "off")
   digitalWrite(13, LOW);

I have attached my block diagram for my app. When I click the LED_ON button nothing happens with the LED. How do I check if I received anything on the serial monitor?

Would appreciate any help. Thanks in advance.

Use Serial monitor for "test prints", debugging, in strategic points. If it is neccesary, cut out other Serial prints in order to catch the debug prints.,

How do I check if I received anything on the serial monitor?

char c = Serial.read(); 

Thank you for sharing this. I noticed that I did not receive anything from the app. Nothing is being displayed in the serial monitor.
I have connected the app with the feather board. But there is no transfer going on.

Start by disconnecting the Bluetooth module. Verify that your code works with commands entered through the Serial monitor. You may have to use no line endings.

If the code works properly, connect the module to the Serial pins of the Arduino. Be sure to cross connect the RX to TX and TX to RX. Then use the phone to send the commands.

If the module/phone to Arduino does not work like the Serial monitor to Arduino you will need further debugging to determine if the app is at fault or the there is a problem with the module.

Does the blink pattern on the module change when the phone is connected?
If you use a standard bluetooth terminal app on the phone to send the commands does the LED turn on and off?