avformat-55.dll missing for exporting in version 2.1.3

I have version 2.1.3 Audacity. I have downloaded FFmpeg version 3.3
We have files that we are trying to convert from G711 to G729. We are receiving them as .wav files. After reading the “How To’s” I thought we would be able to accomplish this with these combinations of tools. I’m not very experienced in working with audio files in this manner, so I hope it’s not something simple I’m doing wrong.
I open the audio file → export Audio → choose Custom FFmpeg Export for the “Save As Type”. That’s when I get prompted that the “avformat-55.dll” is missing.

Any help is greatly appreciated!

Wrong version. See the instructions here: http://manual.audacityteam.org/man/installing_and_updating_audacity_on_windows.html#winff