Yeah, you mixed something up in your template (get_header() twice, a missing closing bracket etc.). Change it to this right here:
<?php
/*
Template Name: Page with custom fields
Template Post Type: ACF
*/
get_header();
?>
<div id="primary" class="content-area">
<main id="main" class="site-main" role="main">
<?php while ( have_posts() ) : the_post(); ?>
<?php get_template_part( 'template-parts/content', 'page' ); ?>
<?php
// If comments are open or we have at least one comment, load up the comment template
if ( comments_open() || get_comments_number() ) :
comments_template();
endif;
?>
<?php
endwhile; // end of the loop.
if (get_field('proprietario')) {
?>
<strong>PROPRIETARIO:</strong> <?php the_field('proprietario'); ?><br />
<?php
}
if (get_field('indirizzo')) {
?>
<strong>INDIRIZZO:</strong> <?php the_field('indirizzo'); ?><br />
<?php
}
?>
</main><!-- #main -->
</div><!-- #primary -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>
The file should be in your theme folder. See if you can select the template now. Here is an explanation on how to create a page template, just for your information (or to compare whether you’ve done everything correctly): https://developer.wordpress.org/themes/template-files-section/page-template-files/#creating-custom-page-templates-for-global-use
