flatpak-builder software package provides Flatpak application building helper, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install flatpak-builder
flatpak-builder is installed in your system.
Make ensure the flatpak-builder package were installed using the commands given below,
$ sudo dpkg-query -l | grep flatpak-builder *
You will get with flatpak-builder package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Development |
| Package | flatpak-builder |
| Version | 0.6.13-1 |
| Description | Flatpak application building helper |
| Sub Section | devel |