I took a look at the repository:
It is noticeable that the instructions recommending gstreamer and vlc-plugins lack a Focal installation candidate - likely the cause of the error.
Interestingingly, it does include not only Focal and even Jammy packages for libde-265, though none specify gstreamer or VLC. Here is that package in .deb format:
https://launchpad.net/~strukturag/+archive/ubuntu/libde265/+files/libde265-0-dbg_1.0.8-1~ppa1~ubuntu20.10.1_amd64.deb
Trusty is Trusty Tahr and that is going back a ways: Ubuntu 14.04.
Remove any repos for Trusty. You can do this from Software & Updates in the other software tab and clicking on mc3man repo to highlight it, then clicking the remove button on the bottom toolbar.
As an aside, here is the full install for gstreamer without any need to add a repository:
sudo apt-get install libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev libgstreamer-plugins-bad1.0-dev gstreamer1.0-plugins-base gstreamer1.0-plugins-good gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly gstreamer1.0-libav gstreamer1.0-tools gstreamer1.0-x gstreamer1.0-alsa gstreamer1.0-gl gstreamer1.0-gtk3 gstreamer1.0-qt5 gstreamer1.0-pulseaudio