You appear to be on Windows (as the error code suggests), which uses the backslash for file paths.
It's not good practice as it won't be widely compatible, but if you're in a hurry and know you won't want to use the code on Mac or Linux, you can use double backslashes:
reader.decode('..\\images\\QR_CODE-easy.png')
Otherwise you should use os.path.join or pathlib (assuming your using Python 3)
import os.path
qr_file = os.path.join("..", "images", "QR_CODE-easy.png")
Or
from pathlib import Path
qr_file = Path("../images/QR_CODE-easy.png")
There are further details of a few options here:
https://medium.com/@ageitgey/python-3-quick-tip-the-easy-way-to-deal-with-file-paths-on-windows-mac-and-linux-11a072b58d5f
Edit: it is also worth confirming that your relative path is indeed correct when starting in your current working directory. You can check the current working directory with: cwd = os.getcwd() . You may want to try an absolute path to your file too, just to confirm whether it works with that first.
More details on cwd here: https://stackoverflow.com/a/5137509/142780