This is related to your setup, as the Utilities class does in fact use the PluginUtilities trait (you can see it in your screenshot there), which is required elsewhere in the plugin.
Your issue may be that the WP Discourse plugin is not being loaded properly and you’re attempting to use the Utilities class in a code snippet. Do you have any custom code using WP Discourse Utilities? Is the WP Discourse plugin currently “Activated”?