NameError: name 'r2_score' is not defined

This error occurs when you try to use the function `r2_score` from the `sklearn.metrics` module, but the module has not been imported or the function has not been defined in the code.

To fix this error, you need to import the `sklearn.metrics` module using the following statement:

from sklearn.metrics import r2_score

Make sure to add this import statement at the beginning of your code.