Undefined variable

Hello,

Thank you for contacting Rank Math, and sorry for any inconvenience caused.

The notice is not an error but a message stating that there are pending Action Scheduler actions that were not completed.

You can follow either of the following steps to remove the warning:

1. Please add the following filter to remove the warning:

add_filter( 'action_scheduler_pastdue_actions_check_pre', '__return_false' );

1.1 Here is a tutorial on how to do it:
https://rankmath.com/kb/wordpress-hooks-actions-filters/

2. Open the Action Scheduler page, click on the Pending tab, Select all and delete them using the Bulk Action dropdown:
https://i.rankmath.com/XBucZ8
Normal WordPress installation path:
/wp-admin/admin.php?page=wc-status&tab=action-scheduler

WordPress installation with WooCommerce in it:
/wp-admin/admin.php?page=wc-status&tab=action-scheduler

3. You can set up a real cron job for the same:
https://www.siteground.com/tutorials/wordpress/real-cron-job/
https://www.google.com/search?q=wp+cron+replace+with+real+cron

Please stay advised that the Action Scheduler will not run if there are no visitors to the site (just like WP’s own cron).

Even Action Scheduler, the library that is used in the RM plugin, states that “Actions that are past-due have missed their scheduled date; because of how WP Cron works, it is normal to have some past-due actions.”

Don’t hesitate to get in touch if you have any other questions.