Hi, I ran into the same error (related to _clock_gettime) today on macos 10.11, but found a workaround. I build ffmpeg using homebrew (brew -v install ffmpeg --with-fdk-aac --with-ffplay --with-freetype --with-libass --with-libquvi --with-libvorbis --with-libvpx --with-opus --with-x265) and then replaced the ChimeraX.app/Contents/bin/ffmpeg with the homebrew ffmpeg. Movies encoding now works in ChimeraX on my macos 10.11. Best wishes Sven The University of Dundee is a registered Scottish Charity, No: SC015096