It’s probably an issue with ffmpeg. You can try downloading an old version which may work better here: https://ffmpeg.zeranoe.com/builds/macos64/static/ffmpeg-3.2-macos64-static.zip. You’ll want to unzip that. Next find the Pencil2D application in Finder and right click on it. There should be an option to “Show package contents” or something like that, click it and then go to Contents/MacOS/plugins. There should be a file called ffmpeg in there. Delete it and copy the ffmpeg file inside the bin folder where you just extracted the newly downloaded zip file.