I had a similar issue to this one on Android 11 with a Pixel 4a, where trying to edit push notification settings for the first time, the error message: "Push Notifications have not been registered for this device." I wanted to create this post to share how I fixed it (I know it's not a question, but I don't see any other knowledge forum on the Canvas Community)
Since it seems like a battery-efficiency related problem, this is what I had to do:
My phone was at 8%
I logged out, disabled adaptive battery in Settings and waited for it to charge
It charged up to 50%
Then, I logged in manually, not using the QR Code.
Then I turned on push notifications.
Finally, I re enabled adaptive battery.
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