Here is my OS information:
NAME="Ubuntu"
VERSION="20.04.1 LTS (Focal Fossa)"

Here is how I am installing: kdesrc-build --include-dependencies kdeconnect-kde
Here is the end of the failure message/log output file:

Code: Select all

...
<<<  PACKAGES SUCCESSFULLY BUILT  >>>
extra-cmake-modules
phonon
taglib
plasma-wayland-protocols
qca
 
Removing 1 out of 1 old log directories...

<<<  PACKAGES FAILED TO BUILD  >>>
kcoreaddons - ~/kde/src/log/2021-01-14-02/kcoreaddons/cmake.log
ki18n - ~/kde/src/log/2021-01-14-02/ki18n/cmake.log
karchive - ~/kde/src/log/2021-01-14-02/karchive/cmake.log
kconfig - ~/kde/src/log/2021-01-14-02/kconfig/cmake.log
kdoctools - ~/kde/src/log/2021-01-14-02/kdoctools/cmake.log
kwidgetsaddons - ~/kde/src/log/2021-01-14-02/kwidgetsaddons/cmake.log
kwindowsystem - ~/kde/src/log/2021-01-14-02/kwindowsystem/cmake.log
polkit-qt-1 - ~/kde/src/log/2021-01-14-02/polkit-qt-1/cmake.log
kcodecs - ~/kde/src/log/2021-01-14-02/kcodecs/cmake.log
kauth - ~/kde/src/log/2021-01-14-02/kauth/cmake.log
kcrash - ~/kde/src/log/2021-01-14-02/kcrash/cmake.log
kdbusaddons - ~/kde/src/log/2021-01-14-02/kdbusaddons/cmake.log
kguiaddons - ~/kde/src/log/2021-01-14-02/kguiaddons/cmake.log
kconfigwidgets - ~/kde/src/log/2021-01-14-02/kconfigwidgets/cmake.log
kitemviews - ~/kde/src/log/2021-01-14-02/kitemviews/cmake.log
kiconthemes - ~/kde/src/log/2021-01-14-02/kiconthemes/cmake.log
kcompletion - ~/kde/src/log/2021-01-14-02/kcompletion/cmake.log
kservice - ~/kde/src/log/2021-01-14-02/kservice/cmake.log
sonnet - ~/kde/src/log/2021-01-14-02/sonnet/cmake.log
attica - ~/kde/src/log/2021-01-14-02/attica/cmake.log
breeze-icons - ~/kde/src/log/2021-01-14-02/breeze-icons/cmake.log
kglobalaccel - ~/kde/src/log/2021-01-14-02/kglobalaccel/cmake.log
ktextwidgets - ~/kde/src/log/2021-01-14-02/ktextwidgets/cmake.log
knotifications - ~/kde/src/log/2021-01-14-02/knotifications/cmake.log
kxmlgui - ~/kde/src/log/2021-01-14-02/kxmlgui/cmake.log
kbookmarks - ~/kde/src/log/2021-01-14-02/kbookmarks/cmake.log
kjobwidgets - ~/kde/src/log/2021-01-14-02/kjobwidgets/cmake.log
kwallet - ~/kde/src/log/2021-01-14-02/kwallet/cmake.log
solid - ~/kde/src/log/2021-01-14-02/solid/cmake.log
kio - ~/kde/src/log/2021-01-14-02/kio/cmake.log
kirigami - ~/kde/src/log/2021-01-14-02/kirigami/cmake.log
kpackage - ~/kde/src/log/2021-01-14-02/kpackage/cmake.log
kparts - ~/kde/src/log/2021-01-14-02/kparts/cmake.log
kactivities - ~/kde/src/log/2021-01-14-02/kactivities/cmake.log
kdeclarative - ~/kde/src/log/2021-01-14-02/kdeclarative/cmake.log
kidletime - ~/kde/src/log/2021-01-14-02/kidletime/cmake.log
kinit - ~/kde/src/log/2021-01-14-02/kinit/cmake.log
kunitconversion - ~/kde/src/log/2021-01-14-02/kunitconversion/cmake.log
oxygen-icons5 - ~/kde/src/log/2021-01-14-02/oxygen-icons5/cmake.log
syntax-highlighting - ~/kde/src/log/2021-01-14-02/syntax-highlighting/cmake.log
kdnssd - ~/kde/src/log/2021-01-14-02/kdnssd/cmake.log
kitemmodels - ~/kde/src/log/2021-01-14-02/kitemmodels/cmake.log
kross - ~/kde/src/log/2021-01-14-02/kross/cmake.log
ktexteditor - ~/kde/src/log/2021-01-14-02/ktexteditor/cmake.log
kwayland - ~/kde/src/log/2021-01-14-02/kwayland/cmake.log
threadweaver - ~/kde/src/log/2021-01-14-02/threadweaver/cmake.log
kded - ~/kde/src/log/2021-01-14-02/kded/cmake.log
kdesignerplugin - ~/kde/src/log/2021-01-14-02/kdesignerplugin/cmake.log
kemoticons - ~/kde/src/log/2021-01-14-02/kemoticons/cmake.log
kfilemetadata - ~/kde/src/log/2021-01-14-02/kfilemetadata/cmake.log
knewstuff - ~/kde/src/log/2021-01-14-02/knewstuff/cmake.log
kpty - ~/kde/src/log/2021-01-14-02/kpty/cmake.log
plasma-framework - ~/kde/src/log/2021-01-14-02/plasma-framework/cmake.log
baloo - ~/kde/src/log/2021-01-14-02/baloo/cmake.log
bluez-qt - ~/kde/src/log/2021-01-14-02/bluez-qt/cmake.log
frameworkintegration - ~/kde/src/log/2021-01-14-02/frameworkintegration/cmake.log
kactivities-stats - ~/kde/src/log/2021-01-14-02/kactivities-stats/cmake.log
kcmutils - ~/kde/src/log/2021-01-14-02/kcmutils/cmake.log
kdelibs4support - ~/kde/src/log/2021-01-14-02/kdelibs4support/cmake.log
kdesu - ~/kde/src/log/2021-01-14-02/kdesu/cmake.log
kholidays - ~/kde/src/log/2021-01-14-02/kholidays/cmake.log
kimageformats - ~/kde/src/log/2021-01-14-02/kimageformats/cmake.log
knotifyconfig - ~/kde/src/log/2021-01-14-02/knotifyconfig/cmake.log
kpeople - ~/kde/src/log/2021-01-14-02/kpeople/cmake.log
kplotting - ~/kde/src/log/2021-01-14-02/kplotting/cmake.log
krunner - ~/kde/src/log/2021-01-14-02/krunner/cmake.log
networkmanager-qt - ~/kde/src/log/2021-01-14-02/networkmanager-qt/cmake.log
prison - ~/kde/src/log/2021-01-14-02/prison/cmake.log
purpose - ~/kde/src/log/2021-01-14-02/purpose/cmake.log
qqc2-desktop-style - ~/kde/src/log/2021-01-14-02/qqc2-desktop-style/cmake.log
syndication - ~/kde/src/log/2021-01-14-02/syndication/cmake.log
pulseaudio-qt - ~/kde/src/log/2021-01-14-02/pulseaudio-qt/cmake.log
kdeconnect-kde - ~/kde/src/log/2021-01-14-02/kdeconnect-kde/cmake.log

The following top 5 may be the most important to fix to get the build to work, listed in order of 'probably most interesting' to 'probably least interesting' failure:

        kcoreaddons
        ki18n
        karchive
        kconfig
        kdoctools

:-(


Why do you try to build something if there are packages available in your distribution? Especially if you try to build the current git version on top of an older Qt version you are pretty much bound to run into problems. Did you check the build instructions e.g. the requirements in the README file?

FWIW: you need to give the actual error messages of the output (the Qt version being the problem is just a guess, but Ubuntu 20.04 IS too old for current git), your output only gives a summary but no indication of what went wrong