You should remove an additional } on line 18. Try the following
<?php
if (!function_exists('understrap_scripts')) {
/**
* Load theme's JavaScript and CSS sources.
*/
function understrap_scripts()
{
// Get the theme data.
$the_theme = wp_get_theme();
$theme_version = $the_theme->get('Version');
wp_enqueue_style('slider-slider-styles', 'https://cdnjs.cloudflare.com/ajax/libs/slick-carousel/1.8.1/slick-theme.min.css');
wp_enqueue_style('lity-style', 'https://cdnjs.cloudflare.com/ajax/libs/lity/2.4.1/lity.min.css');
$css_version = $theme_version . '.' . filemtime(get_template_directory() . '/css/theme.min.css');
wp_enqueue_style('understrap-styles', get_stylesheet_directory_uri() . '/css/theme.min.css', array(), $css_version);
}
} // endif function_exists( 'understrap_scripts' ).
add_action('wp_enqueue_scripts', 'understrap_scripts', 10);