I think you just need to provide a few more arguments to your ffmpeg command and that by default, ffmpeg assumes you want to convert the video to another video format.
I am running MacOS and ffmpeg 4.2.2 as well and am able to create a thumbnail with the following command:
ffmpeg -i sample.mp4 -ss 00:00:01.000 -vframes 1 output.png
Note we tell ffmpeg to take 1 frame (with -vframes 1), from a specific point (-ss <timestamp>) in the video.
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