Thanks everyone,
Bob K.

I've tried the above AUTOMOC settings, and I keep getting the following error:

And did you add qt_wrap_cpp() to moc your mainwindow.h as written by @cristian-adam ? Please post your current CMakeLists.txt

:-1: error: CMakeFiles/Photography.dir/mainwindow.cpp.obj:mainwindow.cpp:(.rdata$.refptr._ZTV10MainWindow[.refptr._ZTV10MainWindow]+0x0):
undefined reference to `vtable for MainWindow'

You will have to either turn off AUTOMOC generally, or specifically for "bad" source files with:

set_source_files_properties(bad_source_file PROPERTIES SKIP_AUTOMOC ON)

But if you are skipping AUTOMOC, you will have to use qt_wrap_cpp for the source files that require moc handling.

Now I'm getting the following error:

:-1: error: CMakeFiles/Photography.dir/Photography_autogen/mocs_compilation.cpp.obj:
in function `MainWindow::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)':

All you have to do, is make sure that you Kit has a Qt Version:

Your CMake project will have in the Initial Configuration -DCMAKE_PREFIX_PATH:PATH=%{Qt:QT_INSTALL_PREFIX}, which will be expanded in current configuration to something like -DCMAKE_PREFIX_PATH:PATH=C:/Qt/6.3.2/mingw_64

Note that there is no lib/cmake/Qt6 or anything like that, just C:/Qt/6.3.2/mingw_64.

That's all that CMake needs in order to find the Qt6 package.

You don't need to change any environment variables, set any QT_DIR CMake variables.

When in doubt just click on the Re-configure with Initial Parameters button, which will clear you CMake configuration and configure the project from scratch.

To be honest, I can't find the options:
projects->Build in qtcreator
Build Environment
Use system environment

I'm not sure where those options are located.

For kits I have:

Auto-detected: Qt 6.3.2 MinGW 64-bit (default)
Manual: n/a

QT_DIR=C:/Qt/6.3.2/mingw_64/lib/cmake/Qt6/lib/cmake/Qt6

There is no Qt5 in that folder.

Here is my CMakeLists.txt file:

cmake_minimum_required(VERSION 3.5)

project(Photography VERSION 0.1 LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Widgets)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Widgets)

#set(app_icon_resource_windows "${CMAKE_CURRENT_SOURCE_DIR}/eesIcon.qrc")

set(PROJECT_SOURCES
        main.cpp
        mainwindow.cpp
        mainwindow.h
        mainwindow.ui
        resources.qrc
)

if(${QT_VERSION_MAJOR} GREATER_EQUAL 6)
    qt_add_executable(Photography
        MANUAL_FINALIZATION
        ${PROJECT_SOURCES}
    )
# Define target properties for Android with Qt 6 as:
#    set_property(TARGET Photography APPEND PROPERTY QT_ANDROID_PACKAGE_SOURCE_DIR
#                 ${CMAKE_CURRENT_SOURCE_DIR}/android)
# For more information, see https://doc.qt.io/qt-6/qt-add-executable.html#target-creation
else()
    if(ANDROID)
        add_library(Photography SHARED
            ${PROJECT_SOURCES}
        )
# Define properties for Android with Qt 5 after find_package() calls as:
#    set(ANDROID_PACKAGE_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/android")
    else()
        add_executable(Photography
            ${PROJECT_SOURCES}
        )
    endif()
endif()

target_link_libraries(Photography PRIVATE Qt${QT_VERSION_MAJOR}::Widgets)

set_target_properties(Photography PROPERTIES
    MACOSX_BUNDLE_GUI_IDENTIFIER my.example.com
    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
    MACOSX_BUNDLE TRUE
    WIN32_EXECUTABLE TRUE
)

install(TARGETS Photography
    BUNDLE DESTINATION .
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR})

if(QT_VERSION_MAJOR EQUAL 6)
    qt_finalize_executable(Photography)
endif()

But you do need to set this QT_DIR for cmake on Windows ENV(not in qt creator env) in case cmake is run from command line to build your project.

QT_DIR=installed qt path/lib/cmake/Qt5

Still not working. I entered the following:

In QT Creator: Edit | Preferences | System | Environment | Change
Entered: QT_DIR=installed qt path/lib/cmake/Qt5

Changed Qt5 to Qt6 and still didn't work.