When testing the endpoint, Magento 2 says "You have
X unprocessed notifications".
There's a problem with the system's cron job.
Make sure that your cron job is running properly.
If the cron job is running, then check the
adyen_notification table if there are unprocessed notificationsmore than 5 days old. Notifications older than 5 days are not processed by cron, and you need to go in the table
adyen_notification in your Magento 2 database and update their creation date to less than 5 days.