Template part including not working

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Your get_template_part call is successful, it’s the file it loads that is broken

<?php get_template_part( 'partials/category' ); ?>

If you swap the contents of that file for this:

<p>Hello World</p>

Then it will work as expected.

The cause of your problem is this:

require_once WP_PLUGIN_DIR.'/themename/includes/file1.php';
  • you should not be including files from other themes directly
  • themes don’t go in the plugins/WP_PLUGIN_DIR folder
  • PHP can’t find the file so it exits with a fatal error

You should see confirmation of this in your PHP error log