Projectopia’s recurring invoices and Projectopia’s subscriptions add-on plugin shares the same wp-cron schedule.
Projectopia subscription add-on will cause a fatal error if you did not set it up correctly.
If you have activated Projectopia subscriptions add on, please deactivate it and see if it fixes your issue.
If it does not work, please proceed to do the following troubleshooting steps, so that we can determine whether it is a WordPress cron issue, or a wrong setting causing fatal error.
1) Please turn on WP_DEBUG and WP_DEBUG_LOG in your wp-config.php
2) Please install the following plugin https://wordpress.org/plugins/advanced-cron-manager/
3) Use this advanced cron manager plugin to trigger pto_check_recurring_invoices cron job
4) You should see the message event has been executed, if not please take note of the cron job error message and let support engineer know.
5) Next check your debug.log file for any fatal error message ( ignore warning and notices ) and let our support engineer know if you see any fatal error message.