excerpt button not going to custom post page

You to set the global $post, otherwise your get_the_ID() function won’t work because this code is not in a WordPress loop.

function all_excerpts_get_more_link($post_excerpt) {
    global $post;

    return $post_excerpt . ' <p><a class="btn btn-secondary understrap-read-more-link" href="'. get_the_permalink( get_the_ID() ) . '">' . __('VIEW CASE', 'understrap')  . '</a></p>';
}

add_filter('wp_trim_excerpt', 'all_excerpts_get_more_link');