"... is noT longer alive" is not equal to "... is no longer alive"?
Your meaning would be understood, but the phrase "is not longer alive" would not be used in English. To use not, you might say something like "is not alive anymore" or "is not living anymore."
What about "Katerina's mother is no longer living"?
That works fine in English, and I have added it.
I entered this very thing (Katerina's mother is no longer living) and it was rejected.
There is no report in the system for it. But "Kateřina's mother is no longer living" tests "green," which means it should have been accepted. If that was exactly what you typed, you may have been bitten by the Duo grading bug.