ffmpeg combining audio and video error: could not find codec parameters

I have downloaded two files using pytube from Youtube. One is audio file .webm extension. While the is adaptive stream 8k video with .mp4 extension. I have to combine audio with videos more than 720p resolution generally. For this I use following FFmpeg code

os.system("ffmpeg -i vid.mp4 -i aud.webm -c copy \""+ title +"\".mkv")

This worked for many mp4 videos I have downloaded. Why it gives error could not find codec parameters for 8k. I dont know much about ffmpeg. If I am doing something wrong please fix this code for the 8k video. The stream output for 8k video is

<Stream: itag="571" mime_type="video/mp4" res="4320p" fps="30fps" vcodec="av01.0.16M.08" progressive="False" 
type="video">

The stream output for webm audio is.

<Stream: itag="251" mime_type="audio/webm" abr="160kbps" acodec="opus" progressive="False" type="audio"> #webm **same for all videos**

The following are the codec I was able to join successfully with same audio

<Stream: itag="135" mime_type="video/mp4" res="480p" fps="30fps" vcodec="avc1.4d401f" progressive="False" type="video">

the other video I was able to merge has the following stream

<Stream: itag="137" mime_type="video/mp4" res="1080p" fps="30fps" vcodec="avc1.640028" progressive="False" type="video">

The question is why ffmpeg giving error and how can this be fixed Note: I am using python to run ffmpeg.

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