Kdenlive snap version doesn't render in Ubuntu 18.04

There may be an issue with the snap package available. Unless this is fixed, you will not be able to use that version. Likely, the regular APT version will work, but as you are still using Ubuntu 18.04, the version that comes with that version will be quite old. Note that Ubuntu 18.04, while still receives maintenance updates, will be EOL next year in 2022.

• If, despite wanting to stick with an older version of the operating system, you want the newest version of KDEnlive, you are in luck. The developpers of KDEnlive package their releases in an AppImage. An AppImage is an application distribution format where all dependencies are contained inside a single file. Therefore, the same AppImage can be run on a number of Linux distributions, including Ubuntu 18.04. Running an AppImage is as simple as downloading the .AppImage file, and running that file after you have set the executable bit (right-click, "Properties" tab in your file manager).
• Another option would be to enable Flatpak on your operating system and install KDEnlive as a flatpak.
• For technically very skilled users, compiling the program yourself based on the source code also remains an option.