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Credit goes to @PieterGoosen

You cannot mix syntax, if you start with if () :, then you need to end with endif, not with a curly bracket. You also cannot use both at the same time like if () : { and then } endif

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
    <header class="entry-header">
        <?php the_title( sprintf( '<h3class="entry-title"><a href="https://wordpress.stackexchange.com/questions/210759/%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h3>' ); ?>

        <?php if ( 'post' === get_post_type() ) : ?>
        <div class="post-details">
          <i class="fa fa-clock-o"></i><time><?php the_date(); ?></time>
        <i class="fa fa-folder-open"></i> <a href=""><?php the_category(', '); ?></a>
        <?php edit_post_link('Edit', '<div><i class="fa fa-penicil"></i>', '</div>'); ?>
                    </div>
        <?php endif; ?>
    </header><!-- .entry-header -->
<?php if (has_post_thumbnail() ) : { ?>
    <div class="post-image">
 <?php the_post_thumbnail(); ?>
    </div>
    <?php } endif ?>

    <div class="post-excerpt">
      <?php the_excerpt(); ?>
    </div>

</article><!-- #post-## -->