Add post excerpt in elements header

I found a possible solution here under the 4th example: https://docs.generatepress.com/article/page-hero-examples/

However, whenever I add that PHP into my functions.php, I get an error. Should I be adding it somewhere else?

I just want the post excerpt to display on my post header created in elements.

I feel like the solution is probably right in front of me, but I'm completely missing it.

Thanks!