Adding a filter on function.php

I followed this tutorial https://docs.generatepress.com/article/adding-local-fonts/, to add some local fonts. On your example is "Open Sans", in my case "Montserrat". I made step by step your guide, but when I add this code on my function.php in generatepress_childtheme, the website doesn't work anymore. Please see the image in the link (https://www.screencast.com/t/AEuD3KbFpSjX), how I added the snippet:

add_filter( 'generate_typography_default_fonts', function( $fonts ) {
$fonts[] = 'Montserrat';

return $fonts;
} );

There is something wrong I did? Thank you!