1, check  ffmpeg permission:

2, check the site setup,  ffmpeg info,

ffmpeg version 4.2-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers  built with gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-6 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg  libavutil      56. 31.100 / 56. 31.100  libavcodec     58. 54.100 / 58. 54.100  libavformat    58. 29.100 / 58. 29.100  libavdevice    58.  8.100 / 58.  8.100  libavfilter     7. 57.100 /  7. 57.100  libswscale      5.  5.100 /  5.  5.100  libswresample   3.  5.100 /  3.  5.100  libpostproc    55.  5.100 / 55.  5.100

3, cronjobd.