NameError

A common error message that usually occurs if you haven’t declared a variable before you use it somewhere in your script.

A Quick Primer on Variables in Ren'Py
In this tutorial for beginners, you'll learn about booleans and how to set up variables in Ren'Py.
The Definitive Default Define Ren'Py Article
In this tutorial for beginners, learn how to declare variables in Ren'Py to track data which will be saved and loaded.