Could not locate full support for the H.264 video codec; video playback support may be less reliable than expected.
Likely you are looking for the RPMfusion multimedia packages - see Making sure you're not a bot!
It is also possible that you are in a region that has restricted access to the fedora-cisco-openh264 repo.
Fedora ships with a package noopenh264 and is designed to, on the first update after installation, replace that with the openh264 package. If your system does not have the openh264 package installed then you will not be able to play media that used the h264 codec.
To fine out if you have the openh264 package already installed please run dnf list --installed \*openh264. That should show either openh264 or noopenh264 in the results.
If you do not have the openh264 package installed then try updating from the command line in a terminal to see the results.
sudo dnf update and watch the messages to see what it says about openh264 and the fedora-cisco-openh264 repo.