Hi,
I'm new with Arduino. What is wrong with my Serial communication?
Both with the online compiler and the IDE I don't receive any message from Serial Monitor.
I've already programmed the board, so it's not a problem of the board.
Please don't post pictures of text. That is very unhelpful.
On the boards with native USB like your Leonardo, the microcontroller doesn't reset when you open Serial Monitor. This means that "75" will have already been printed before you even get Serial Monitor open. For this reason, it's common to add this code to your sketch before the first Serial print:
while(!Serial);
The program will pause at this line until Serial Monitor is opened. Note that you would not want to have this line if you wanted your program to run even without Serial Monitor being open.