What does apt policy libkf5textedittexttospeech1 show?
The package libkf5textedittexttospeech1 installs fine for me here, though I don’t have anything installed that uses it. I wonder if your package database is not up to date, perhaps, or just a temporary server hiccup.
If you don’t see a difference, try using apt update and apt full-upgrade, which will show more thorough messaging, since pkcon hides a lot of this when it runs apt.
Your hint took me to the idea to install the package “libkf5textedittexttospeech1” separately via apt before I repeated to update the whole system, which has been executed fully successful now.
It seems that apt wasn’t able to install this package automatically which was not installed before I tried to run the system update.