Checking the build log:

> cat /home/chris/kde/src/log/2024-02-04-01/kiconthemes/build.log
# kdesrc-build running: 'ninja' '-j' '4'
# from directory: /home/chris/kde/build/kiconthemes
[0/1] Re-running CMake...
CMake Deprecation Warning:
  Support for "Extra Generators" like

    Kate

  is deprecated and will be removed from a future version of CMake.  IDEs may
  use the cmake-file-api(7) to view CMake-generated project build trees.


--

Installing in /home/chris/kde/usr. Run /home/chris/kde/build/kiconthemes/prefix.sh to set the environment for KIconThemes.
CMake Error at /home/chris/kde/usr/lib/cmake/KF6BreezeIcons/KF6BreezeIconsConfig.cmake:32 (include):
  include could not find requested file:

    /home/chris/kde/usr/lib/cmake/KF6BreezeIcons/KF6BreezeIconsTargets.cmake
Call Stack (most recent call first):
  CMakeLists.txt:62 (find_package)


CMake Warning (dev) at /home/chris/kde/usr/share/ECM/modules/ECMQmlModule6.cmake:49 (message):
  The CLASSNAME argument to ecm_add_qml_module is deprecated for Qt 6; Use
  CLASS_NAME instead.
Call Stack (most recent call first):
  src/qml/CMakeLists.txt:1 (ecm_add_qml_module)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- The following features have been enabled:

 * DESIGNERPLUGIN, Build plugin for Qt Designer

-- The following OPTIONAL packages have been found:

 * OpenGL
 * XKB (required version >= 0.5.0), XKB API common to servers and clients., <http://xkbcommon.org>
 * Qt6GuiTools (required version >= 6.7.0)
 * WrapVulkanHeaders
 * Qt6Quick (required version >= 6.5.0)
 * Qt6CoreTools (required version >= 6.7.0)
 * Qt6DBusTools (required version >= 6.7.0)
 * Qt6QmlTools (required version >= 6.7.0)
 * Qt6WidgetsTools (required version >= 6.7.0)
 * Qt6Test (required version >= 6.5.0)

-- The following REQUIRED packages have been found:

 * ECM (required version >= 5.249.0), Extra CMake Modules., <https://commits.kde.org/extra-cmake-modules>
 * Qt6Svg (required version >= 6.5.0)
 * Qt6Core (required version >= 6.5.0)
 * KF6Archive (required version >= 5.249.0)
 * Gettext
 * KF6I18n (required version >= 5.249.0)
 * Qt6DBus (required version >= 6.5.0)
 * Qt6Qml (required version >= 6.5.0)
 * Qt6Widgets (required version >= 6.5.0)
 * KF6ConfigWidgets (required version >= 5.249.0)
 * KF6WidgetsAddons (required version >= 5.249.0)
 * KF6ColorScheme (required version >= 5.249.0)
 * Qt6UiPlugin
   Required to build Qt Designer plugins
 * Qt6

-- The following features have been disabled:

 * QCH, API documentation in QCH format (for e.g. Qt Assistant, Qt Creator & KDevelop)

-- Found clang-format version 16
-- Configuring incomplete, errors occurred!
FAILED: build.ninja
/usr/bin/cmake --regenerate-during-build -S/home/chris/kde/src/kiconthemes -B/home/chris/kde/build/kiconthemes
ninja: error: rebuilding 'build.ninja': subcommand failed

P.S. Problem resolved following these instructions!!