[FFmpeg-user] error in Codec AVOption b

[Michele Salerno]

I have this command:

ffmpeg -y -hide_banner -threads 8 -hwaccel nvdec
-hwaccel_output_format cuda -i INPUT.mp4 \
 -vf scale_cuda=-1:360 -c:v h264_nvenc -rc vbr -minrate 400k -maxrate
1500k -bufsize 1500k \
 -acodec aac -strict experimental -ar 44100 -b:a 64k \
 -preset slow -flags +loop -cmp chroma -subq 6 -me_range 16 -g 250
-keyint_min 25 -sc_threshold 40 -i_qfactor 0.71 -b_strategy 1 \
-map 0:v -map 0:a? OUTPUT.mp4

I have this messages:
Codec AVOption b (set bitrate (in bits/s)) specified for output file
#0 has not been used for any stream. The most likely reason is either
wrong type (e.g. a video option with no video streams) or that it is a
private option of some encoder which was not actually used for any
stream.

In OUTPUT-mp4 the audio is stripped
Because?

Thanks.