transcode cannot find encoding device

[ywangwxd]

I am using the ffmpeg 6.1. Although the compiling of this source code has some small issues, I can fix it and make it compiled. However, the example program always complain error message about h264 encoder.

[h264_v4l2m2m @ 0x557cb57e20] Could not find a valid device
[h264_v4l2m2m @ 0x557cb57e20] can't configure encoder

The decoder is working well. As for the h264 encoder, I am sure it does exists on my ffmpeg. It is in the list generated by the command ffmpeg -codecs, such as,

DEV.LS h264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (decoders: h264 h264_v4l2m2m) (encoders: h264_v4l2m2m)

If my understanding is correct, it shows that my ffmpeg support h264 decoding and encoding. What is more, I can do transcode using ffmpeg command, so the encoding looks OK. Why it still complains error?