@silver-sun
I think Mint 21.3 is based on Ubuntu 22.04
qt6ct won't build in 22.04, but maybe you could try
https://software.opensuse.org//download.html?project=home%3APeterBBB&package=qt6ct
(Use Grab binary packages directly) and install with gdebi
or here
https://packages.ubuntu.com/lunar/qt6ct
again, use gdebi. package will not install if dependencies are not satisfied.