I see that libvo_aacenc is set as the default encoder in this file: https://github.com/eprints/eprints/blob/3.3/perl_lib/EPrints/Plugin/Convert/Thumbnails.pm

But from discussion online, I see that libvo_aacenc became unsupported (https://askubuntu.com/questions/483187/winff-ffmpeg-unknown-encoder-libvo-aacenc)

This causes an error when creating a video preview (on Ubuntu 18.04)
"Unknown encoder 'libvo_aacenc'" and the video preview fails to generate.

Instead of installing an unsupported encoder, I changed line 189 and line 201 in https://github.com/eprints/eprints/blob/3.3/perl_lib/EPrints/Plugin/Convert/Thumbnails.pm to switch to “aac” from “libvo_aacenc”. It solved the issue.