This error means that Pencil2D could not run the ffmpeg plugin. Make sure that in the same directory as the pencil2d.exe file there is a directory called plugins with a file called ffmpeg.exe in it. If you do not have this file in this exact location, you did not install Pencil2D correctly and you should reinstall it, making sure to extract all files from the zip download.
If you do have that file, make sure that your system or antivirus software isn’t blocking it from running.
There are some generic tips for working around Windows’ quirks here, some of which may help with your issue if the suggestions mentioned above are not enough to fix it.