It might happen that when you upload a sketch - after you have selected your board and the serial port -, you get an error Error opening serial port ... If you get this error, you need to set serial port permission. Below are the steps on how to set the serial port permission.

Steps

1. Open Terminal and type ls -l /dev/ttyACM* you will get something like:

   crw-rw---- 1 root dialout 188, 0 5 apr 23.01 ttyACM0

   Note: The "0" at the end of ACM might be a different number, or multiple entries might be returned. The data we need is "dialout" (is the group owner of the file).

2. Now we just need to add our user to the group, use the following command in the terminal, and change with your Linux user name.

sudo usermod -a -G dialout <username>

You will need to log out and log in again for this change to take effect.

After this procedure, you should be able to normally and upload the sketch to your board or use the Serial Monitor.