gst-inspect-1.0 avdec_h264 says no such element, but gstreamer1.0-libav 1.16.2 is already installed

[wanted002]

    Still, I can see libgstlibav.so at
/usr/lib/x86_64-linux-gnu/gdstreamer-1.0/ . And ffmpeg is installed too.
I've no idea...Please help me with some advices~ Thanks.
    My laptop: ubuntu 20.04LTS ,  gstreamer plugins ,sunch as good, bad,
base and so on are all installed...



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