@Sebastien-Thiry
Seems like the version of motion you are using does not support direct video output . A simple workaround is to let motion output image series and then use ffmpeg to compile the images into a video afterwards.

Here is a some what related tutorial on capturing time-lapse video with Omega2 hope it helps
https://docs.onion.io/omega2-project-book-vol1/timelapse-camera.html

Here is how the final video looks like
https://vimeo.com/230518247