Kdenlive does not work out of the box after install.
Package qml-module-qtquick-controls is missing.
Open a terminal (CTRL + ALT + t) and copy and paste:
sudo apt -y install qml-module-qtquick-controls
After that it works fine.
Also found a couple of bug reports.
https://bugs.launchpad.net/ubuntu/+source/kdenlive/+bug/1641192
https://bugs.launchpad.net/ubuntu/+source/kdenlive/+bug/1634478