The wp_cron job has not run in a while and might be disabled. Until you fix this, transactions can be delayed. Triggering a cron run now. Check the accompanying PDF manual for ways to debug and solve the issue.
You are seeing this error message either because your site has low traffic or because your cron job is not running for some reason. If the cause is low traffic, then you can safely ignore this message. If something else is wrong, and no transactions are being executed, then please follow the instructions given in troubleshooting guide (part of the PDF documentation), and please contact me if the problem persists or if you have any further questions.