This project is outdated.
Use the latest 5.8 kernels and enable rkvdec (VIDEO_ROCKCHIP_VDEC) in the staging drivers, for good support on RK3399 boards.
You can also try Kwiboo's kernel branch here : https://github.com/Kwiboo/linux-rockchip
For FFMPEG support, try this branch : https://github.com/Kwiboo/FFmpeg/commits/v4l2-request-hwaccel-4.2.2