The video created by this command:
ffmpeg -y -framerate 25 -i picture.png -i sound.wav -vcodec libx264 -crf 20 -preset medium -acodec aac -vf scale=1280:-2,format=yuv420p output.mp4 is not accepted ("Video assets: Your mezzanine file has failed.") by Amazon Prime.
How to make a video accepted by such a third party?
Your video input is a single frame, which lasts 0.04 seconds. Likely the cause of your submission failure. Loop the image indefinitely and then tell ffmpeg to stop output file with shortest stream (the audio).
ffmpeg -y -framerate 25 -loop 1 -i picture.png -i sound.wav -vf scale=1280:-2,format=yuv420p -vcodec libx264 -crf 20 -preset medium -acodec aac -shortest -fflags +shortest -max_interleave_delta 200M output.mp4
