See: https://flathub.org/apps/org.kde.kdenlive
Support for older NVIDIA GPUs has been removed with PR#253 (see also PR#259).
We have kept support for those older GPUs in the Beta branch.
$ flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
$ flatpak install flathub-beta org.kde.kdenlive
Note that this branch will likely not get any further updates.