Server error (5xx) and Call to Undefined Function

We have an error in our Google Search Console. It says it’s a Server Error (5xx) for https://www.fairlesshillscosmeticdentist.com/wp-content/plugins/wp-video-lightbox/misc_functions.php. That is the only URL listed. I looked in the plugin files. There’s an error_log file there. Since March 6th it’s been saying PHP Fatal error: Call to undefined function add_shortcode() in (our folder structure)/wp-content/plugins/wp-video-lightbox/misc_functions.php on line 2.

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