I have never done dnf update, but I suspect you are getting this message because the current updates are now installing kernel V5.0.
On Thu, 2019-05-09 at 18:21 +0200, Andreas Aßmann wrote:
When trying to update by typing "dnf update" I always get the error message

"""
Error:
Problem: The operation would result in removing the following protected
packages: kernel-core