'Requested entity was not found' error - MobilePush Sends not received by Android devices

Device Tokens associated with registrations for inactive Android devices that have not connected to Firebase Cloud Messaging (FCM) in over a month will be considered stale.

Tokens can become stale from lack of use (accessing your application) or deleting/uninstalling your application.

After 270 days of inactivity, Firebase Cloud Messaging (FCM) will consider these stale tokens as expired, and mark them as invalid, which rejects any future sends targeting that Android device.

To confirm if a particular DeviceID in an errored send has a stale token, you can utilize the MobilePush Detail Extract Report via Automation Studio to gather the reporting for that particular MobilePush Message.

Sends targeting Android devices with a Stale or Expired token will result in the following Status & Service Response:

Status : Not Found
Service Response : Requested entity was not found

Likewise you can review the individual MobilePush Device associated with the Contact record within Contact Builder to determine if their token has expired due to lack of activity by following the steps below:

1. Navigate to Contact Builder, select 'All Contacts' tab.
2. On the left-hand side, under the 'Channel' section, select 'MobilePush'
3. In the Search bar, search the ContactKey or affected DeviceID, select the Contact record
4. Select the 'Attributes' tab, navigate to 'MobilePush Data' section
5. Within 'MobilePush Demographics' locate the DeviceID to see the 'Last Application Open' attribute value 

After three (3) sends targeting this particular DeviceID result in this error, Marketing Cloud will automatically mark this DeviceID as opted out.