My advice is to give some thought to how your devices will connect to the Blynk server in the long term.
Clearly, using the USB serial connection via a computer which has to be running and have the command prompt open 24/7 isn’t the solution.
If your ultimate connection method will be hard-wired Ethernet then you don’t have many choices, and you’ll struggle to get a reliable long-term solution.
If your ultimate connection method will be WiFi then you have many more choices. You could go down the route of using an ESP-01 as a WiFi modem for an Arduino Uno/Mega, but I really wouldn’t recommend it.
Instead I’d suggest using an ESP8266 or ESP32 instead of the Arduino Uno/Mega, as they both have built-n WiFi capabilities, far more memory, and the ability to update them via OTA.
That way, instead of spending money on an FTDI adapter to allow you to see what messages your Mega is giving you about why your serial USB connection is failing, you’ll be able to spend that money on the device that you will ultimately use in your long-term project.
Pete.