It’s important to remove any traces of the fake ffmpeg that Ubuntu unfortunately packages. You know you have it because if you run ffmpeg you get a vindictive message from former developers who left the project.
- Here is the info: askubuntu.com/questions/313217/install-latest-ffmpeg-for-all-users-in-debina-ubuntu
The official installation guide seems to be maintained regularly:
Some problems and solutions are here: