I see the problem here, to make encoding working, you should install the package intel-media-va-driver-non-free instead of the free version. Otherwise it will show error:
ffmpeg: /build/intel-mediasdk-mEneEL/intel-mediasdk-19.4.0/_studio/shared/src/mfx_vpp_vaapi.cpp:239: mfxStatus MfxHwVideoProcessing::VAAPIVideoProcessing::Init(void**, mfxVideoParam*): Assertion `0x00000000 == vaSts' failed.
It seems that the encoding part of the free version driver got a problem. Any chance to fix this?