Other possibilities
You’re calling the function before ACF is loaded. I don’t see how this is possible given the code you’re provided. If ACF is loaded it would be so before the “the_content” hook. Unless wp-content/themes/jacksons/functions.php on line 102 is running before ACF is loaded.
You have a corrupt or missing ACF plugin file. I also don’t see how this is possible as it should cause other errors during WP startup. However you could try reinstalling ACF, this wouldn’t hurt anything.
