Note: this error is unlikely, unless if you are using a custom theme, coded by an inexperienced developer.

Check if your theme is calling the wp_footer() function. Typically this function will be called in your theme’s footer.php file.

The conditional fields javascript code is loaded during wp_footer, so a call to this function is crucial. If there is no such call in your theme, go to your theme’s footer.php file and add this code right before the closing </body> tag:

<?php wp_footer(); ?>