This usually happens when testing an ad-hoc/enterprise build of the iOS App to test if notifications are working. The most likely cause is that your the notification was sent via the wrong gateway due to a misconfiguration.
Apple have two notification gateways one for sending to dev builds (sandbox) and the other for sending to production builds. To test with a production build you will need to have a production push notification cert uploaded in App Settings > Certs & Keys.
You should generate this type of cert:
And in upload it here:
As described in docs here: