The Connect notification category is missing on user's system setting.
There isn't an active notification for the "Connect" notification category which can be checked under sysevent_email_action table.
1) Navigate to sysevent_email_action.list
2) Check to make sure that there is at least one active notification that uses the "Connect" category
This URL can be used to check for it: /sysevent_email_action_list.do?sysparm_query=category.nameSTARTSWITHconnect
More info on notification category: https://docs.servicenow.com/bundle/jakarta-servicenow-platform/page/administer/notification/task/create-notification-categories.html