This is a PC / Arduino issue, not directly Blynk related. Google avrdude: ser_open(): can’t open device to see the various steps other Arduino users have tried when resolving their own COM port / driver issue.
Basically, either you are setting your IDE to the wrong COM port, or you have a serial port driver issue? Perhaps more likely, based on the ser_open() part, you have some other program already communicating to COM 16 when you try to upload (e.g. Blynk’s USB .bat or script file?), etc.