Hey @Cj19
The 'Operation Failed' when changes Notifications settings on the APP could due to APP not syncing to correct time zone after checking with our engineer.
Log into your Kasa APP. Tap “3 lines” icon in the top left corner> Settings> Location & Time, select time zone and sync location
How to Adjust Time Setting in the Kasa APP
It would be nice to also check if app & device firmware are both up to date.
Please check if router firewall blocks any tp-link domain, or restart Wi-Fi router and see if it helps.
Can I have a photo of the error message, the model and OS on the phone, the APP version. Feel free to let us know for any further observations,thank you!