DrAzzy:
That error message has nothing to do with Serial.print() - that's an error from the upload or bootloading process. Do you have an Arduino with jtag2updi installed on it connected to the ATtiny1614?
I know that and yes I have an Arduino Uno with jtag2updi connected to the ATtiny1614.
DrAzzy:
You cannot upload via the serial port unless you are using the (optiboot) board definition, and have used a UPDI programmer (like jtag2updi) to install the bootloader.
I know know, but I don't have an optiboot.
I successfully loaded basic sketches like the blink, following this tutorial https://www.youtube.com/watch?v=AL9vK_xMt4E I need to print something on the serial monitor and I found out I can use a serial adapter, but I don't know how to use it and I can't find a simple guide about it.