How to implement Flask flash Function

I'm hosting a website locally and I'm having trouble implementing the flash functionality in Flask, to display messages to the user interface if they have logged in and if they have logged out, I've been watching a few tutorials to try n fix my problem but I keep getting a 500 Internal Server Error when I refresh the website, but don't get the error when I erase the code in my html n py files. I've been making use of Flask logics in my html files n been extending my header file to all the files, so I don't know why I'm getting this error.

Anybody to assist??