@m @kdenlive If you are using a self compiled version it then you must enable the qt6 flags for Kdenlive and MLT. See build instructions: https://invent.kde.org/multimedia/kdenlive/-/blob/master/dev-docs/build.md
If you are using a distro package then request the Debian packagers to build Kdenlive with Qt6.
Another alternative could be to use the Appimage or Flatpak versions or switch to Archlinux (or Arch-based) distros.