name 'arff' is not defined

This error message usually means that the "arff" module has not been imported or installed in your Python environment.

To install "arff" module, you can use pip, the package installer for Python. Open your command prompt or terminal and type the following command:

pip install liac-arff

Once the installation is complete, you can import the "arff" module in your Python code using the following command:

import arff

This should resolve the error message you are seeing.