Hi @elkswebteam,
The function referenced is wp_json_encode(), which is WordPress core’s wrapper around PHP’s native json_encode() function. wp_json_encode() was introduced in WordPress 4.1.0, back in late-2014(!).
Looking at the <meta name="generator" /> tag on your site’s source, it appears that your site is currently running version 4.0.12 of WordPress, which pre-dates the introduction of wp_json_encode().
I’d recommend updating to the most recent release of WordPress as soon as you’re able, as there have been a number of major security fixes released since your last update. Once upgraded, I would suggest re-installing the WP101 Plugin (assuming you’re a subscriber), as wp101.php is responsible for bootstrapping the plugin.
Be forewarned, however, that WordPress core introduced its new block-based editor (a.k.a. “Gutenberg”) in version 5.0, so the content editing experience has changed. Should you need more time to get authors up-to-speed, the WordPress team maintains a Classic Editor plugin to restore the older functionality.
In the meantime, I’ll make sure that the “Requires at least” is updated within the WP101 Plugin to reflect that version 5.x of the plugin indeed requires at least WordPress 4.1.
