ok, got it.
It looks like all the requirements were not installed in the VM.
I got it running good by:
- doing an sudo apt install openjdk-8-jre libwebkitgtk-1.0-0 build-essential python-jinja2 ninja-build zip autoconf automake (as suggested on https://source.sierrawireless.com/resources/airprime/software/legato_application_development_kit_linux/)
- delete the workspace folder
- start DS again
It should probably help.