Display option doesn’t work

Hello,

With the Nisarg Theme, the display option doesn’t work. I use this code :

<?php
$post_display_option = get_theme_mod( 'post_display_option', 'post-excerpt' );
if ( 'post-excerpt' === $post_display_option ) {
	get_template_part( 'template-parts/content', 'excerpt' );
} else {
	get_template_part( 'template-parts/content', get_post_format() );
}
?>

Any indication how can I do this?
Thanks a lot!