On Firebase:

• Goto Settings > Project Settings > General

• Scroll down to find the iOS app

• Add Team ID & App Store ID

• GoTo Cloud Messaging Tab & make sure you've uploaded the authentication keys (p8 file). The same is generated from Developer Console > Keys.

Also, make sure you have added the Team ID & App Store ID as well.

On Xcode:

• Enable Push Notification
• Enable Background Modes & select Remote Notifications and Background fetch

I had faced the same issue on iOS and the authentication key(p8) file was missing & also the team IDs and App Store Ids were missing, adding which solved the problem for me.