@Jordan_River_Studios is right that ffmpeg is an external program used for importing and exporting some formats in Pencil2D. However, you should not download it from other websites. Pencil2D is only compatible with specific versions of Pencil2D, and for Windows especially it can be tricky to find the correct builds. Additionally if you download it from a bad source, it could conceivably contain malware, although I am not aware of any instances of this being a problem with ffmpeg downloads so far.
It is for these reasons, and for ease-of-use, that we include ffmpeg with the Pencil2D zip download. If you navigate in your file explorer to the directory where Pencil2D is installed (ie. the one with the pencil2d.exe file), there should be a plugins folder in that same directory. Open that up and you should see a “ffmpeg” file. If you don’t see this (and you probably won’t given the error you’re encountering), you need to add it. To do this, download the Pencil2D zip file and either: 1. “Reinstall” Pencil2D by extracting all the contents of the zip file, making sure that all files including ffmpeg are extracted, and replacing your Pencil2D install directory with these files, or 2. Extract just the ffmpeg file from the plugins directory in your zip file, and move it to the plugins folder in your Pencil2D install directory.
Sometimes Windows will block extracting this file or antivirus programs will quarantine it which can lead to this issue, so keep an eye out for that. If you are still having troubles getting Pencil2D installed correctly, you may find some of the tips in this guide helpful: [Guide] How to Train Your Pencil2D (Windows)