Ggtcf

First the problem:
My code is broken if I add an if alternative statement inside another if statment in a loop:

if ( has_post_thumbnail( $post->ID ) ) : ?>
 <li>
 <?php $partner = the_field('industry_partner_links'); ?>
 <?php 
 if ($partner) :
 ?>
 <a href="<?php the_field('industry_partner_links'); ?>" target="_blank">
 <?php echo get_the_post_thumbnail( $post->ID, 'full' ); ?>
 </a>
 <?php 
 else :
 ?>
 <a href="#" target="_blank">
 <?php echo get_the_post_thumbnail( $post->ID, 'full' ); ?>
 </a>
 <?php 
 endif;
 ?>
 </li>

My goal is to create a loop in my template to display some image (they have a URL that will direct to an image) based on a custom post in a Wordpress environment BUT, if the image doesn't have the link (ACF), the url will be empty (href=#").

Essentially, I created a section with my sponsors (image wrapped in a link) and I feed those images and URLs from the backend of WordPress using ACF (advanced custom field). Spoiler, it works.

Tha basic code in php:

<?php 

$args = [
 'posts_per_page' => -1,
 'order' => 'ASC',
 'orderby' => 'title',
 'post_type' => 'partner',
 'post_status' => 'publish',
];

if ( $posts = get_posts( $args ) ) 
 echo '<ul class="feed-industry-partners">';
 foreach ( $posts as $post ) 
 setup_postdata( $post ); 
 if ( has_post_thumbnail( $post->ID ) ) : ?>
 <li>
 <a href="<?php the_field('industry_partner_links'); ?>" target="_blank">
 <?php echo get_the_post_thumbnail( $post->ID, 'full' ); ?>
 </a>
 </li>
 <?php endif;
 wp_reset_postdata();
 
 echo '</ul>';


?>

Any Idea why it is not working?

From the code you gave here I can guess that function the_field('industry_partner_links') does not return anything but it prints to output. So good news, your code is correct. The problem is with logic here. Variable $partner = the_field('industry_partner_links'); will be always falsy. But there is a workaround with output buffering (but this is ugly solution): <?php ob_start(); the_field('industry_partner_links'); $product = ob_get_flush(); ?>. More proper way is to make some switch in function the_field() which will return value instead of printing it. Or maybe make another function which will do the same job but it will return the result, hm? :)