A new application needs to be pushed to all iOS devices.

The error message “NotNow” is seen in the Event Log on an iOS device's details page when an action cannot be performed because the device is locked with a passcode. These actions include pushing managed apps, installing profiles, and other actions. When this occurs the device will attempt to re-connect with the MDM server as soon as the device is unlocked in order to retry the action. https://documentation.meraki.com/SM/Monitoring_and_Reporting/Status_of_%22NotNow%22_in_Systems_Manager_Event_Log