When you type

in the terminal, it is assumed that the file myfile.py is already available in the current directory, and that myfile.html will be created in that same directory.

When you open up a new terminal window, the current directory will by default be your home directory, also known as ~ or $HOME.

However, you say that you can "see the file on your desktop", which likely means that the file is in fact in a desktop directory somewhere beneath your home directory, i.e. the file is not directly available in the home directory.

You then have a few possible alternatives.

Change directory in the terminal to where the Python script is actually located. If the desktop directory is ~/Desktop, then

will take you there. You may then run the python3 command as above. The myfile.html file will be created in the Desktop directory.

Move the file into your home directory. Provided that the file is located in the ~/Desktop directory, you may

which will move the file. You may then proceed with the python3 command as above. The myfile.html will be created in the home directory.

Run the Python script with an complete path from the home directory:

The myfile.html file will be created in the home directory.