The apt.kitware.com for at least focal is broken currently, looks like cmake-data 2.24.0 is premature and missing. Using focal-rc works with 2.24.0~rc5 so it is specific to the “stable” version of the repository.
sudo apt install cmake
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
cmake : Depends: cmake-data (= 3.23.2-0kitware1ubuntu20.04.1) but 3.24.0-0kitware1ubuntu20.04.1 is to be installed
E: Unable to correct problems, you have held broken packages.
Our build server is currently in the process of building CMake for several architectures and Ubuntu versions. Sometimes the cmake-data package becomes available before the cmake package, because cmake-data has the all architecture, so it only needs to be built once, whereas the cmake package needs to be built for every architecture. Please be patient - the rest of the packages should be available later today.
The same problem has happened again (cmake : Depends: cmake-data (= 3.24.0-0kitware1ubuntu20.04.1) but 3.24.1-0kitware1ubuntu20.04.1 is to be installed). It looks like the cmake package requires an unspecified version of cmake-data, hence getting its latest version instead of the matching one.
I worked around this by installing cmake-data beforehand while stating the same version, e.g. export CMAKE_VERSION=3.24.0-0kitware1ubuntu20.04.1 && apt-get -y install --no-install-recommends cmake-data=$CMAKE_VERSION cmake=$CMAKE_VERSION.