By default, the_excerpt(), doesn’t uses the post/page content wysiwyg, it has its own textarea, that by default is not visible, you need to enable it first via screen options.
After you checked this checkbox you can scroll down to the bottom of the post/page and see a new textarea, this is the excerpt.
If you want to use the content (wysiwyg, the_conetnt() or get_the_content()) as excerpt, you will need to use wp_trim_words.
Something like this.
// the second argument is how many words to trim, default is 55
<?= wp_trim_words(get_the_content(), 30); ?>
Anothe option, if you want to keep the html structure would be to do this.
<?= force_balance_tags(html_entity_decode(wp_trim_words(htmlentities(wpautop(get_the_content())), 30))); ?>