Not sure HN is the best forum for debugging such issues, but nevertheless, here goes:
# lsb_release -a
Ubuntu 20.04.5 LTS
# qmake --version
QMake version 3.1
Using Qt version 5.12.8 in /usr/lib/x86_64-linux-gnu
# qmake-qt5 --version
qmake-qt5: command not found
# apt-file search qmake-qt5
qt5-qmake-bin: /usr/share/man/man1/qmake-qt5.1.gz
# /usr/lib/qt5/bin/qmake --version
QMake version 3.1
Using Qt version 5.12.8 in /usr/lib/x86_64-linux-gnu
I don't recall doing anything special to install Qt on my box, likely something along the lines of:
# apt-get install qt5-default
So in all likelihood, I have the standard Qt5 that comes with Ubuntu 20.04.5, still a somewhat popular and widely used Ubuntu version.
In other news, I tried compiling it on a server that runs a recent version of Ubuntu (22.04) and it does compile and seem to compile and link against Qt 5.15.3
Standard and traditional Qt type headaches, where between many different major versions and deprecated stuff and newly introduced APIs between minor versions, compiling and linking software that relies on Qt is a complete crapshoot.
Also, since you don't provide statically linked binary releases or a flatpak or an appimage, you might want to consider making the source dependencies of your software explicit in the Readme (like e.g. the minimum version of Qt5 required, or the qt5Charts dev libs, definitely not something everyone has installed on their computer).
Hope this helps.