An incorrect fix for a bug in the rpm-ostree backend in Discover introduced an issue preventing updates from being offered to the user in the interface in Plasma 5.27.5 (bug).
This has been fixed in Plasma 5.27.6 (fix) which is now in Fedora Kinoite 38.
As this issue is preventing applying system updates in Discover, you will have to manually update your system using the command line:
$ rpm-ostree update
$ reboot
You should then be able to update using Discover again.