Hi, on the Help/Scheduler page you should see the specific error: if the job is just late, you need an external trigger, as explained on this page:
https://www.thenewsletterplugin.com/documentation/troubleshooting/newsletter-delivery-engine/#external-system
The error message takes a while to disappear but you can see on the Help/Scheduler page the graph reporting a shorter interval between the scheduler activation (it should be less than 300 seconds).
If you have a cache system like CloudFlare wrongly configured (caching the calls to the wp-cron.php calls) you should ask the provider to fix it. If you share your site address I can try to check that problem.
If you have an error “job skipped”, this is a more serious issue and needs to be debugged.
https://www.thenewsletterplugin.com/documentation/troubleshooting/newsletter-delivery-engine/#job-skipped