NameError: name 'QtCore' is not defined

This error usually occurs when the QtCore module is not imported correctly in your Python code. You can try importing it using the following line of code:

from PyQt5 import QtCore

If you are using a different version of PyQt, replace PyQt5 with the appropriate version (e.g. PyQt4).

Alternatively, you can import the entire PyQt5 library using the following line of code:

import PyQt5

This will import all modules in the PyQt5 library, including QtCore.