FileNotFoundError: [Errno 2] No such file or directory

If the user does not pass the full path to the file (on Unix type systems this means a path that starts with a slash), the python file path is interpreted relatively to the current working directory. The current working directory usually is the directory in which you started the program. In order to make this work, the directory containing the python executable must be in the PATH, a so-called environment variable that contains directories that are automatically used for searching executables when you enter a command.

In any case, if your Python script file and your data input file are not in the same directory, you always have to specify either a relative path between them or you have to use an absolute path for one of them.