When using the USB link, it is important that you follow a few guidelines. First, you cannot use the IDE serial monitor at the same time as the USB script… as the IDE monitor uses the primary UART (
Serial)… the same one your Mega is communicating to the Blynk Server over.
That is the repeating Auth code you see as it keeps broadcasting that until it is recognised and the connection links up.
In fact you shouldn’t even be able to open the IDE monitor when the USB script is running (and it MUST be running in order to connect).
If you need serial output, then you need a TTL-USB adapter, hooked up to
Serial1 on the Mega (pins 18 & 19) and another terminal program for viewing on the PC. I use Termite.
Then, once you are properly connected, and if your sensor is wired correctly, you should see the proper values in the App.
I would also recommend changing this line…
timer.setInterval(3000L, sendSensor); // Run sendSensor() function every three seconds
to give that sensor enough time (3 seconds) to process between reads… it is really slow.